Hundreds of thousands of people have been displaced since March 2015, when clashes between supporters of Yemen’s president and northern rebels pushed the country into civil war.
President Abd-Rabbu Mansour Hadi fled to Riyadh in March, where he set up a government in exile. That same month, a Saudi-led coalition began air strikes on the rebel forces. The United Nations is trying to broker talks between the warring parties.
The escalation in fighting has worsened an already severe humanitarian crisis. It has disrupted imports to Yemen, and fuel shortages have crippled hospitals and food supplies. About 12 million people do not have enough to eat, the United Nations said in May.
At the end of last year an estimated 10.6 million people were food insecure, 840,000 children under five acutely malnourished, and more than 13 million had no safe water and basic sanitation. Overall, some 15.9 million Yemenis out of a population of 25.9 million needed humanitarian aid.
The United Nations says civilians are bearing the brunt of the fighting, with reports of hospitals, schools, water systems, mosques and airports coming under attack. Aid agencies are hampered by the fighting and fuel shortages.
Many people die every year trying to make the dangerous sea journey from the Horn of Africa to Yemen. In 2014, the country was host to more than 245,000 refugees, most of them from Somalia.
But Somalis and Yemenis are now fleeing in the opposite direction, and the U.N. refugee agency (UNHCR) said in April it was planning for a "worst case scenario" of 130,000 refugees to arrive in the Horn of Africa within six months.
The government has also been fighting southern separatists for many years, and faces a threat from al Qaeda militants.
Yemen is the poorest Arab nation, and its economic hardships have been aggravated by a lengthy political crisis as well as conflict on several fronts. It has high levels of unemployment, rapid population growth and diminishing water resources.
It is heavily dependent on dwindling oil supplies, but its oil and gas pipelines have been repeatedly sabotaged by insurgents and tribesmen, causing fuel shortages and slashing export earnings.
Fighting in the northern provinces between the Houthis and the government has raged on and off since 2004, and recently spread south. It has displaced hundreds of thousands of people.
The Houthis seized the capital Sanaa in September 2014 after leading weeks of protests there. They imposed informal control of government ministries, and demanded that the government step down and that fuel subsidy cuts imposed in July be reversed.
They signed a U.N.-brokered peace deal with 11 political parties in September, stipulating that a new unity government be formed. But the Houthis and Hadi did not fully implement the deal.
The Houthis expanded their territory southward and along the Red Sea coast, and took the lead in the fight against al Qaeda militants. They said they were providing security until the government could carry out its responsibilities, according to the International Crisis Group (ICG).
By the end of 2014, they were the dominant authority in nearly half of the country’s 21 governorates, and had representatives in ministries and other state institutions.
The Houthis rejected a draft constitution proposed by the government in January 2015, and seized the presidential palace in Sanaa. The draft was the result of talks between Yemen’s competing factions. The Houthis opposed the draft’s proposal to divide Yemen into six federal regions, and instead supported a plan by southern separatists for just two regions.
When they tightened their grip on the capital in January, Hadi and his government resigned, and Hadi was put under house arrest. Hadi later re-claimed the presidency.
He fled Sanaa in February, and tried to set up a rival power centre in the south with loyalist army units and tribes. In March he fled to Riyadh where he set up a government in exile.
Houthi and Saudi forces have exchanged fire on both sides of the border with Saudi Arabia.
WHO ARE THE HOUTHIS?
The Houthis are based in Yemen’s northern highlands and named after the clan of their leader. They are seeking more rights for the Zaydi Shi'ite Muslim sect, a strongly tribal minority in mostly Sunni Muslim Yemen, and want greater autonomy for their heartland of Saada province. They oppose Yemen's close ties with the United States.
The Houthis are allied with Shi’ite Iran and backed by forces loyal to former president Ali Abdullah Saleh who was forced to step down in 2011. Their alliance with Saleh is tense, the two sides having fought six wars against each other, ICG said.
Saudi Arabia, Yemen’s largest donor, sees the Houthis as proxies of rival Iran and accuses Tehran of providing them with weapons, a charge Iran denies.
Supporters of the Houthis praise their willingness to confront corruption, combat al Qaeda and fill a security vacuum left by the government, ICG’s April Longley Alley said earlier this year.
Those opposed to them include both southern separatists and people who want to maintain Yemen as a single state. The former resent President Hadi, a southerner himself, who is pro-unity.
In the far north, hundreds of thousands of people have been displaced by fighting in the last few years between the Houthis and government forces.
Fighting escalated in 2009, and the conflict briefly spread to Saudi Arabia where rebels clashed with Saudi forces, accusing the government there of supporting the Yemeni government in attacks against them. The Saudi government denied this.
Despite a shaky ceasefire deal with the government in February 2010, most of the displaced were reluctant to return home because of continued insecurity, a fear of reprisals and a lack of basic services.
The Houthis have clashed with northern followers of Salafism, a puritanical brand of Sunni Islam, and with supporters of the powerful Islamist Islah Party.
Separatists in the south have intensified calls for an independent state for the south, alarmed at the growing chaos in northern Yemen.
The separatists want to revive a southern socialist state that Saleh united with the north in 1990. They initially joined the anti-Saleh protesters in 2011, but the two sides later moved apart.
Yemen's unity was troubled from the start and resulted in a short but bloody civil war in 1994. After that many southerners viewed the north as an occupying force, and their sense of marginalisation by the government eventually resulted in a popular protest movement in 2007, which later made calls for separation, ICG said.
Many southerners complain that northerners based in the capital Sanaa have discriminated against them and usurped their resources for decades. Most of Yemen's fast-declining oil reserves are in the south. The central government denied allegations of discrimination.
Southern Yemen has also been affected by a nascent Islamic State movement and al Qaeda-linked militants.
The latter are expanding in the south, taking advantage of anti-Houthi feelings and state collapse, ICG said.
Al Qaeda took similar advantage of a power vacuum in 2011, seizing several towns in Abyan governorate, including the governorate capital Zinjibar which was nearly destroyed in clashes between government forces and militants.
The militants carried out public summary executions, amputations and flogging during their rule, and the military used excessive force as it fought back, Amnesty International said.
When the situation in Abyan worsened, local tribes began to turn against the Islamists and joined government forces in an effort to push them back, ICG said.
Saleh held up the Islamists' sudden success as an example of what al Qaeda would do if Yemen slipped further into chaos. But opponents suspected he had given a signal to militants, with whom the government was in contact, to make their move because he felt it would strengthen his case with the West for political survival.
In May 2012, after Saleh had stepped down, the government launched a major U.S.-backed offensive and regained control of the towns in June 2012.
The Islamist militants include Ansar al-Shari'a (Partisans of Islamic Law) and al Qaeda in the Arabian Peninsula (AQAP).
AQAP was formed when the Saudi and Yemeni branches of al Qaeda merged in 2009. It is based in Yemen.
The United States has been using drones to strike the Islamist militants, but discontent at their use has been growing. Armed tribesmen have held protests, saying the strikes kill innocent civilians, and some say they are an infringement of sovereignty.
Mass protests began in early 2011 demanding the end of President Ali Abdullah Saleh’s 33-year rule and these, combined with growing violence by Islamist and tribal militants, meant the government lost control of chunks of the country for several months.
Tens of thousands of people were displaced in southern, central and northern areas. Public services in most of the country stopped, and the unrest created severe fuel shortages and dramatically pushed up the price of food.
The number of malnourished children under the age of five nearly doubled to 750,000 as a result of the political turmoil, the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said in February 2012.
The 2011 protests against Saleh paralysed the country. Some of his top aides, military commanders, cabinet ministers and diplomats defected to the protesters' side.
Houthis carved out their own domain in the north, and al Qaeda militants seized towns in the south.
Fighting involving government forces and rebels, Islamist militants and rival tribesmen affected several parts of the country, mainly the capital Sanaa and the southern and northern regions.
Tribesmen attacked and closed the main oil pipeline in the oil-producing Maarib province, near Sanaa, in 2011. They blockaded the area, costing the government millions of dollars a day in lost exports and sparking a severe fuel crisis, power outages and soaring prices.
After surviving an assassination attempt in June 2011, Saleh went to Saudi Arabia for medical treatment. Under a deal brokered by the Gulf Cooperation Council, he finally agreed in November to hand power to his deputy, Vice President Abd-Rabbu Mansour Hadi, and a unity government was formed.
The deal brought some calm to Sanaa, but violence continued in other parts of the country, especially the governorates of Sa'ada and Hajjah in the north, and Abyan and Aden in the south.
In January 2012 parliament granted Saleh full immunity from prosecution, and a month later presidential elections were won by Hadi, the sole candidate.
The country ended a 10-month National Dialogue Conference involving many different groups in January 2014. Although it produced principles for a new constitution, it failed to generate consensus on power-sharing arrangements, and the future of the south.
The Yemen Times gives news in English, with detailed reports of clashes and sieges.
The BBC profile includes links to other media.
For the latest U.N. Office for the Coordination of Humanitarian Affairs reports, visit ReliefWeb.
The Carnegie Endowment for International Peace has published research on Yemen.
Human Rights Watch regularly monitors the situation in Yemen.
London-based think tank Chatham House also produces occasional briefing papers on Yemen.
The U.N. Children's Fund (UNICEF) tracks the plight of children in conflict-affected areas as well as the country as a whole.
The U.N. Refugee Agency (UNHCR) monitors refugees, and theInternal Displacement Monitoring Centre monitors those displaced by Yemen's internal conflicts.
The World Food Programme (WFP) has useful information about Yemen's food needs.