DAMASCUS (AFP) ― 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 Organization of Islamic Cooperation.

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 U.N. figures has killed over 60,000 people in nearly two years ― the U.N.-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 U.S., which said it was in talks with the Syrian opposition about opening offices to represent it in Washington and New York.

For the Syrian opposition coalition this is part of a drive to take the country’s U.N. seat from President Bashar al-Assad’s government, its new U.S. representative said Wednesday.

Khatib has also received an invitation from the U.S. government to visit Washington and could seek meetings with top U.N. officials, Coalition envoy Najib Ghadbian said.

“The Assad regime has lost its legitimacy, so we have the goal of taking over the Syrian seat at the U.N.,” Ghadbian said, while acknowledging that it would be a “long legal and political battle.”

The U.S. recognized the opposition coalition as the legitimate representative of the Syrian people late last year, and has been working with it to help oust Assad.

But the Syrian regime signaled it believes it can still break the military stalemate, as its forces relentlessly pounded rebel lines around Damascus.

“The army has launched a coordinated all-out offensive on all of the areas surrounding the capital,” said a Syrian security official who declined to be identified.

“All entries to Damascus have been sealed.”

Rami Abdel Rahman of the Syrian Observatory for Human Rights said the bombardment and clashes around Damascus were the heaviest for a long time.

“The province was bombed very badly in attacks that have not been seen in months. There was also very heavy fighting,” he said.

In the south-central city of Palmyra, two suicide car bombings targeting a military intelligence headquarters killed 19 members of the security forces, the Observatory said.

The watchdog said at least 122 people -- including 32 civilians -- were killed nationwide on Wednesday.

Meanwhile, a court in Lebanon ordered the extradition of a Syrian officer-turned-rebel who entered the country illegally, with rights groups warning he faces death if handed over to Damascus.