DAMASCUS: Syria's army Wednesday launched a fierce assault in the Damascus region, shelling rebel neighbourhoods and sealing main entrances to the capital, witnesses said, as prospects for peace talks dimmed.

Shattering months of relative calm in Damascus, the 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 came as an offer by opposition leader Ahmed Moaz al-Khatib for peace talks with President Bashar al-Assad's regime suffered sharp 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.

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

"These things cannot wait until judgment day," Khatib told BBC Arabic.

"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," he said.

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.

Khatib defended his decision to meet Iranian Foreign Minister Ali Akbar Salehi, a move the SNC criticised, while also appealing for the opposition to "declare our willingness to negotiate" the regime's departure.

Addressing the OIC summit in Cairo, Egypt's President Mohamed Morsi alluded to the divided opposition, calling on 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."

Khatib announced last week he was ready for talks with the regime, subject to conditions including the release of 160,000 detainees. He later said he was ready to meet Vice President Faruq al-Sharaa.

In the past the opposition has demanded Assad step down before talks can begin, but analysts say Khatib's change in stance stems from a belief the population will be bled dry while the West fails to act.

Assad's regime signalled that it believes it can still break the military stalemate of the past few months, 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," he told AFP.

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

"The province was bombed very badly in attacks that have not been seen in months. There was also very heavy fighting," said Abdel Rahman.

Residents said "unusually intense bombing" was heard throughout the capital all day.

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, 51 rebels and 39 troops -- were killed nationwide on Wednesday. The UN says more than 60,000 people have died in violence since the uprising started in March 2011.