An Islamist suicide car bomber killed at least 50 Syrian security men in Hama province on Monday, an opposition group said, in what would be one of the bloodiest single attacks on President Bashar al-Assad's forces in a 20-month-old uprising.
Another day of relentless violence in Syria coincided with more unity talks in Qatar among fractious opposition factions.
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.
Rami Abdelrahman, head of the Syrian Observatory for Human Rights, said the centre was used by security forces and pro-Assad militia as one of their biggest bases in the area.
"A fighter from the Nusra Front blew himself up," he said. "He drove his car to the centre and then blew himself up. A series of explosions followed. At least 50 were killed."
The Nusra Front, an al-Qaeda-inspired group of ultra-orthodox Salafi Muslims, has claimed responsibility for several suicide bombings in Damascus and elsewhere in the past. It operates mostly independently of other rebel factions, some of which have criticised it for indiscriminate tactics.
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."
Syrian officials often blame foreign-backed Islamist militants for the anti-Assad revolt, in which about 32,000 people have been killed.
Warplanes, tanks and artillery battered rebel-held parts of southern Damascus in what one Western diplomat said was a major escalation in Assad's campaign to crush rebels. Opposition activists said at least 10 people were killed there.
An air strike on Haram, a town in the northwestern province of Idlib near the Turkish border, killed at least 20 rebels of the Idlib Martyrs' Brigade, probably including their commander, Basil Eissa, the Syrian Observatory said.
Much of Idlib province is in the hands of insurgents, but remains vulnerable to air power, used increasingly by Assad's forces to contain his mostly Sunni Muslim opponents.
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, 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 this week.
Discussions focused on a proposal by influential opposition figure Riad Seif for a new structure combining the rebel Free Syrian Army, regional military councils and other insurgent units with local civilian bodies and prominent individuals.
Unity on Syria has also eluded major international powers since the conflict 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, but Western nations have fought shy of supplying anti-tank or anti-aircraft missiles without a credible opposition leadership.
That has given the Syrian military a free hand, with densely populated Damascus suburbs hit by air and ground 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), a 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.
The Syrian conflict has aggravated divisions in the Islamic world, with Shia Iran supporting Assad and U.S.-allied Sunni nations such as Saudi Arabia, Turkey and Qatar backing his foes.
Russian 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," Mr. Lavrov was quoted as saying.
After talks with his Egyptian counterpart in Cairo, Mr. Lavrov said Russia 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.
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 turned from peaceful protests into a civil war.