* Battle breaks relative lull
* Opposition says parts of strategic ring-road captured
* Security forces under attack in eastern city Palmyra (Adds Salehi, Alkhatib comments)
By Khaled Yacoub Oweis and Mariam Karouny
AMMAN, Feb 6 (Reuters) - Syrian rebels battled President Bashar al-Assad's forces on the edge of central Damascus on Wednesday, opposition activists said, seeking to break through to the heart of the capital.
Their offensive aims to break a stalemate in the city of 2 million people, where artillery and air strikes have prevented rebels entrenched to the east from advancing despite their capture of army fortifications, the activists said.
"We have moved the battle to Jobar," said Captain Islam Alloush of the rebel Islam Brigade. The district links rebel strongholds in the suburbs with the central Abbasid Square.
"The heaviest fighting is taking place in Jobar because it is the key to the heart of Damascus," he said.
Assad, battling to crush a 22-month-old uprising in which 60,000 people have died, has lost control of large parts of the country but his forces, backed by air power, have so far kept rebels on the fringes of the capital.
Despite the setbacks, Assad has remained defiant, telling a visiting senior Iranian official on Sunday that Syria was able to confront "current threats ... and aggression".
That visit came just after Iranian Foreign Minister Ali Akbar Salehi met Syrian opposition leader Moaz Alkhatib in Moscow. Iran is Syria's strongest backer in the region.
Salehi said on Wednesday the Syrian government may be ready to respond positively to Alkhatib's offer of direct peace talks.
"I think that the Syrian government is ready to negotiate with the opposition," he told Egyptian state news agency MENA during a visit to Cairo.
But Alkhatib gave the Syrian government until Sunday to release all women detainees, otherwise he would regard his offer of dialogue as rejected by Assad.
"If any woman stays in prison, I consider the regime not responding," BBC Arabic quoted him as saying.
Meanwhile Syrian state media denied rebel gains in Jobar and said the army had pushed back rebels from the neighbourhood and other parts of the Ghouta area of eastern Damascus.
"Our noble army is continuing its operations against the terrorists in Irbeen, Zamalka and Harasta and Sbeineh, destroying the criminal lairs," Syrian television said.
But video footage taken by activists purported to show opposition fighters inside Jobar after they overran an army road block, and rebels said they had made significant gains.
"Parts of the Damascus ring road fell to us today. The road has been effectively the last remaining barrier between the Ghouta and the city," said Abu Ghazi, a rebel commander based in the eastern suburb of Irbeen.
"I don't want to give people false hopes but I think if street fighting reaches central Damascus, the regime will not be able to quell it this time."
A disorganised rebel advance on the city failed last year. Ghazi said that this time opposition fighters had established supply lines to support their offensive.
The Damascus Media Office, an opposition activists' monitoring group, said 13 people had been killed in fighting in Jobar, while three people had died in army shelling on Thalatheen, a rebellious neighbourhood in southern Damascus.
The Syrian National Council, an exile opposition group dominated by the Muslim Brotherhood, said Syrian Free Army rebel units were attacking "strategic targets" in Damascus.
"There is a new strategy, brigades are united. What is happening in the field is huge but it is a preparation for bigger operations," said Abu Moaz al-Agha, a leader of the Gathering of Ansar al-Islam, which groups many Islamist units.
"Right now we will attack checkpoints especially in Jobar that some time ago seemed impossible to get near to. We want to shake the regime."
Abbasid Square and the Fares al-Khoury thoroughfare were closed as fighters attacked roadblocks and fortifications with rocket-propelled grenades and mortars, activists said.
"The areas of Jobar, Zamalka, al-Zablatani and parts of Qaboun and the ring road have become a battleground," activist Fida Mohammad said from Qaboun.
Residents reported explosions across the east and north of the capital. "The army seems to have been caught by surprise," one activist said. "Reports from the heart of the battle are talking about several tanks being hit and the army has been pushed to Abbasid Square."
The rebel Liwa al-Islam unit said the operation to enter eastern parts of Damascus aimed to relieve pressure on two large southwestern suburbs that have been under army siege.
Assad's core forces, mostly from his minority Alawite sect, an offshoot of Shi'ite Islam, are based in Qasioun Mountain, which is part of Damascus, and on hilltops dotted with artillery pieces and multiple rocket launchers.
Estimated at 70,000 army, security and militia personnel, the core forces have a supply line to the coast that has remained open despite rebel efforts to disrupt it.
Rebels were also attacking Adra, 17 km (10 miles) northeast of Damascus. Video footage purported to show an armoured vehicle in the area being hit by a rocket. Thousands of refugees have fled to the town, which is home to Syria's largest prison.
In Palmyra, 220 km (140 miles) northeast of Damascus, on the main road to the oil-producing east, a suicide car bomb struck a military intelligence compound, causing dozens of casualties, opposition campaigners said.
A bomb destroyed part of the back wall of the compound near the Roman-era ruins in the city and then a suicide car bomber drove through, detonating the vehicle and destroying parts of the facility, activists in Palmyra said.
They said it was not immediately clear how many people had been killed in the blast and the clashes that followed. Video footage, which could not be immediately verified, showed a large cloud of thick smoke rising in the city.
"The first car bomb struck at around six in the morning. The second one, which caused the larger explosion, broke through into the compound 10 minutes later," activist Abu al-Hassan said from the city.
He said tanks in the compound had responded by shelling an adjacent neighbourhood, killing several civilians.
Roadblocks across the city also came under attack.
The state news agency said two "suicide terrorists" blew up cars packed with explosives near a garage in a residential district, killing and wounding several people.
Street demonstrations against Assad's rule erupted in Palmyra at the beginning of the revolt almost two years ago. But the army has since tightened its control of the city, which is situated near a major oil pipeline junction. (Editing by Kevin Liffey and Jon Hemming)