* Simmering tensions and violence in capital Bangui
* Some 20 killed in attack on Muslim convoy near western town of Bouar
* Interim government due to appoint new president on Monday
* U.N. official calls for bigger international response (Adds death toll, attack on Muslim convoy, paragraphs 15-17)
By Emmanuel Braun and Tom Miles
BANGUI/GENEVA, Jan 16 (Reuters) - A senior U.N. official warned of the risk of genocide in Central African Republic without a more robust international response to communal bloodshed in which at least 20 more people were killed on Thursday.
The former French colony descended into chaos after a mostly Muslim rebel coalition, Seleka, seized power in March, unleashing a wave of killings and looting that sparked revenge attacks by Christian militia known as "anti-balaka" (anti-machete).
More than a million people have been displaced by the violence since Seleka installed their leader Michel Djotodia as interim president. Over 1,000 people were killed last month alone in the capital Bangui, prompting neighbouring countries to evacuate more than 30,000 of their citizens.
There has been relative calm since Djotodia resigned last week under intense international pressure, but sporadic violence has persisted in Bangui. On Thursday, a spokesman for a 15,000-strong group of anti-balaka criticised the interim government and threatened a return to violence if it was not overhauled.
"If there is no solution to this, we always have our machetes which we have not yet handed in," the spokesman, Sebastien Wenezoui, told Reuters at a base in the northern Bangui suburb of Boeing, flanked by around 20 militiamen armed with knives, machetes and Kalashnikovs.
His group wants the country's transitional assembly (CNT) to be reworked to boost the presence of the anti-balaka and reduce the number of Seleka representatives. It plans a march in Bangui on Friday to try to stop the appointment of a new interim president, he added.
The CNT is due to vote on the new head on Jan. 20, its vice president Lea Koyassoum Doumta said on Thursday.
SEEDS OF GENOCIDE
Central African Republic is designated by the United Nations as one of the top three global humanitarian emergencies, along with Syria and the Philippines. But a U.N. appeal has received only 6 percent of a $247 million target.
Returning from a five-day trip to the country, a U.N. official said the crisis was foreseeable and stemmed from many years of international neglect of the chaotic country.
"The elements are there, the seeds are there, for a genocide," John Ging, director of operations for the U.N. Office for Coordination of Humanitarian Affairs (OCHA), told a news conference in Geneva. "It has all the elements that we have seen elsewhere, in places like Rwanda and Bosnia."
Ging said the country amounted to little more than a territory on a map, without state infrastructure and functioning security forces. He said French and African Union (AU) peacekeepers were having a positive effect but were stretched to their limits.
"Central African Republic has to move up the priority list," he said. "However desperate and alarming the situation might be right now, it can be turned around very quickly."
France hurriedly sent some 1,600 troops to its former colony in December. The deployment of Rwandan troops, the first of whom arrived aboard a U.S. military aircraft on Thursday, will increase the AU contingent to more than 5,000 peacekeepers this month.
Many say the bloodshed has little to do with religion, in a nation where Muslims and Christians have long lived in peace. Instead, they blame a political battle for control over natural resources in one of Africa's weakest states, split along ethnic lines and worsened by foreign meddling.
Around 20 more people were killed on Thursday when anti-balaka fighters attacked a convoy of Muslims, many of them women and children, headed for the town of Bouar, some 450 km (280 miles) to the west of Bangui, witnesses said.
The families were fleeing militia attacks in the nearby town of Bohong.
"There were about 20 lifeless bodies strewn here and there on the road. Some were killed by bullets, others with knives," Achille Yakende, a salesman travelling to Bouar by motorbike, told Reuters by telephone.
In Bangui, hundreds of angry residents surrounded a convoy of Chadian Muslims fleeing the country. They shouted "go home" and "assassins". Some in the crowd tried to loot and set fire to one vehicle but were prevented by AU peacekeepers, who fired warning shots to disperse them.
At least eight people were killed late on Wednesday in violence in the north of the capital, witnesses said.
The commander of French forces, General Francisco Soriano, told a video conference in Paris the security situation was improving, but still "extremely complicated and very volatile." (Additional reporting by Paul-Marin Ngoupana and Emmanuel Braun in Bangui; John Irish in Paris; Writing by Bate Felix and Emma Farge; Editing by Mark Trevelyan, Daniel Flynn and Eric Walsh)