Syria's army has launched a fierce assault around Damascus, blasting rebel strongholds and sealing the main entrances to the capital, witnesses said, as prospects for peace talks dimmed.

Shattering months of relative calm in the capital, Wednesday's offensive coincided with two suicide car bombings that killed almost 20 members of the security forces in the ancient city of Palmyra, a watchdog reported.

It also came as an offer by opposition leader Ahmed Moaz al-Khatib for peace talks with President Bashar al-Assad's regime suffered setbacks, with Damascus ignoring it and a key opposing faction flatly rejecting the initiative.

The surprise gesture by Khatib, head of the opposition National Coalition, was welcomed by the United States and the Arab League, and was expected to receive the backing of the Organisation of Islamic Cooperation (OIC).

International Syria mediator Lakhdar Brahimi welcomed the "positive" offer but warned it was not enough for a political solution.Asked if he could see an end in sight to Syria's bloodshed -- which according to UN figures has killed over 60,000 people in nearly two years -- the UN-Arab League envoy told the French daily La Croix: "Not for the moment."

Assad himself has yet to comment on the offer by Khatib, who stepped up the pressure to engage in talks by setting the regime a deadline of Sunday for the release of all women held in Syrian prisons.

"The demand that the women are released means that if there is one single woman still in prison in Syria on Sunday, I consider that the regime has rejected my initiative," Khatib told BBC Arabic.

The Syrian National Council, the main component of the Coalition, has rejected the possibility of any talks, saying it is committed to ousting Assad's regime, rejecting dialogue with it, and protecting the revolution.

But Khatib, who last week called on Assad to agree to let Vice President Faruq al-Sharaa open peace talks with his coalition, appealed for the opposition to "declare our willingness to negotiate" the regime's departure.

Addressing an OIC summit in Cairo, Egypt's President Mohamed Morsi called on opposition factions "to coordinate with this coalition and support their efforts for a unified approach... for democracy".

According to a draft OIC resolution obtained by AFP, the summit is to call for "serious dialogue" between the opposition and government officials "not directly involved in oppression".

It stresses the need to maintain "Syria's territorial integrity and sovereignty", while underlining "the main responsibility for the continued violence falls on the Syrian government".

That echoes the strong backing Khatib received from the US, which said it was in talks with the Syrian opposition about opening offices to represent it in Washington and New York.