An Islamist suicide car bomber killed at least 50 Syrian security men on Monday, an opposition group said, in what would be one of the bloodiest single attacks on President Bashar al-Assad's forces.

Syrian state media reported that a suicide bomber had targeted a rural development centre in Sahl al-Ghab in Hama province, but put the death toll at two.

"A fighter from the Nusra Front blew himself up ... At least 50 were killed," said Rami Abdelrahman, the head of the Syrian Observatory for Human Rights. "He drove his car to the centre and then blew himself up. A series of explosions followed."

Abdelrahman, whose monitoring group is based in Britain, said the rural development centre was used by Syrian security forces as one of their biggest bases in the area.

The state news agency Sana said: "A terrorist blew himself up in the centre which resulted in a number of casualties. Two citizens were killed and a number of them were wounded."

The Nusra Front is an Islamist group made up of militant Salafis, or ultra-orthodox Muslims. It has claimed responsibility for several suicide bombings in the past.

meanwhile Syrian warplanes and artillery battered rebel strongholds in southern Damascus on Monday in an onslaught that coincided with another attempt by Syria's fragmented opposition to unite.

A Western diplomat said the assault in Damascus marked a major escalation in a campaign by President Bashar al-Assad's forces to quell opposition in the capital's Sunni Muslim areas.

The bombardment, unleashed hours after a rebel attack on a pro-Assad militia, killed at least 10 people, activists said.

In Qatar, divided Syrian opposition groups were meeting to try to forge a cohesive leadership that would then make common cause with rebel factions fighting on the ground, in an effort to gain wider international recognition and arms supplies.

The Syrian National Council (SNC), the largest overseas-based opposition group, was expected to expand its membership to 400 from 300 and to elect a new leader and executive committee before talks with other anti-Assad factions in Doha on Thursday.

Unity on Syria has also eluded major international powers since the revolt against Assad began in March 2011, with Russia and China opposing Western calls for his removal and critical of so far ill-coordinated outside efforts to arm his opponents.

Rebels have few weapons to counter warplanes and artillery, used increasingly to contain anti-Assad forces. Densely populated Damascus suburbs have taken the brunt of bombardments that have killed hundreds of people in the last three weeks.

Witnesses said artillery deployed on Qasioun, a mountain that overlooks Damascus, was pounding southern neighbourhoods and warplanes were firing rockets. Tanks were also in action.

Activist Rami al-Sayyed, speaking from southern Damascus, said rebels had made hit-and-run attacks on pro-Assad militiamen in the city overnight before retreating to the nearby farmland of al-Ghouta, or the old gardens of Damascus.

"The rebels are avoiding their past errors of trying to hold onto territory, where they would be crushed. They are waging a war of attrition, hitting regime forces quickly and retreating to the rear," he said.

In one attack, rebels fought pro-Assad militiamen in Nisreen, a southern district mainly populated by members of Assad's minority Alawite sect, an offshoot of Shi'ite Islam.

They also hit positions belonging to the Popular Front For the Liberation of Palestine-General Command (PFLP-GC), Syrian-sponsored faction, in the nearby Yarmouk refugee camp, where at least 20 people were reported killed by army shelling on Sunday.

At least seven PFLP-GC members were killed in the latest fighting, and ambulances were seen taking dozens of casualties from Nisreen to hospital, activists in the area said.

In eastern Syria, the Jaafar bin Tayyar Division, a rebel unit in Deir al-Zor, said it had taken control of the al-Ward oilfields near the Iraqi border on Sunday, after overrunning a loyalist outpost defended by 40 militiamen.

But rebel commanders, former Syrian officials and the Syrian head of an oil services company said the fields, mostly not operational, had been under de facto rebel control for months.

The Syrian conflict has aggravated divisions in the Islamic world, with Shi'ite Iran supporting Assad and US-allied Sunni nations such as Saudi Arabia, Turkey and Qatar backing his foes.

Russia's Foreign Minister Sergei Lavrov told Egypt's al-Ahram daily that Moscow, Syria's main arms supplier, was sending weapons under Soviet-era commitments for defence against external threats, not to support Assad.

"We do not side with any faction in Syria's internal battle," Lavrov was quoted as saying.

After talks with his Egyptian counterpart in Cairo, Lavrov said Russia said backed an Egyptian initiative that seeks to bring together Egypt, Saudi Arabia, Turkey and Iran to try to resolve the Syrian crisis. Saudi Arabia has stayed away from the last two meetings of the disparate regional group.

Egypt, Saudi Arabia and Turkey have all said Assad must leave power. Iran advocates dialogue to resolve the crisis.

In Cairo, peace envoy Lakhdar Brahimi called on Sunday for a UN Security Council resolution based on a deal major powers reached in June to set up a transitional Syrian government.

But Lavrov, speaking at the same news conference, said this was unnecessary and that others were stoking violence by backing rebels. His comments highlighted the impasse over Syria.

Russia and China, both permanent Security Council members, have vetoed three Western-backed UN draft resolutions condemning Assad's government for its handling of an uprising that began with peaceful protests, but turned into a civil war in which an estimated 32,000 people have been killed.-Reuters