At least 30 civilians were reported killed on Thursday in a massive explosion in northeast Syria, AFP reported.
State media blamed a helicopter gunship crash on an accident but monitors said rebels shot it down, according to the report.
Dozens more people were wounded in the blast at a petrol station in the northeastern village of Ain Issa, the Syrian Observatory for Human Rights said. Activists said it was caused by an air strike.
"At least 30 people were killed and 83 were injured, although unconfirmed sources say the number of dead was actually more than 50," Observatory director Rami Abdel Rahman told AFP.
"Lawyers and activists in the area say the blast was caused by aerial bombardment," the Britain-based watchdog added.
Activists said the petrol station, in Raqa province, was hit by a warplane.
The deaths came a day after Syrian rebels seized the Tal al-Abyad border crossing between Raqa province and Turkey in heavy clashes with regime troops.
The blast at Ain Issa was some 25 miles south of the border post, noted AFP.
The military helicopter that went down outside Damascus crashed after an accident with a civilian aircraft, AFP quoted state television as having said.
"This morning's helicopter crash resulted from an accident in the air when the helicopter's rotor clipped the tail of a Syrian Air plane carrying 200 passengers," it said, citing air traffic controllers at Damascus airport and Syrian Air as saying the airliner landed safely.
The Observatory reported the helicopter was downed by rebels following a series of explosions in the restive town of Douma, northeast of Damascus.
Clashes raged on Thursday as the Observatory, which relies on the accounts of activists on the ground, said the death toll in the 18-month uprising had surpassed 29,000 people, the vast majority civilians.
Fierce clashes raged in rebel-held Talbisseh and Rastan in the central province of Homs, it said.
In eastern Aleppo a doctor said at least 12 civilians were killed, including children, while shops in the Old City remained closed.
A rebel commander named as Abdel Basset al-Ahmadi was also killed in fighting on Thursday, the Observatory said.
It said at least 154 people were killed nationwide on Thursday, including the 30 in Raqa.
Meanwhile, AFP reported, diplomats from more than 60 nations and the Arab League meeting in the Netherlands urged the UN Security Council to choke off the resources Assad's regime needs to battle the uprising.
The "Friends of Syria" working group on sanctions called on the world and "particularly members of the UN Security Council" to implement measures to deny Damascus access to resources "for its campaign against its own people."
The group, which previously met in Tunis, Istanbul and Paris, plans to gather again in Morocco in October and later in Italy, the report said.
Meanwhile, Assad on Thursday slammed Saudi Arabia, Turkey and Qatar for arming the rebels.
"The widespread idea that Saudi Arabia, Syria and Egypt are the cornerstone of stability in the region is false. It has always been, and will remain, Syria, Iraq and Egypt," he told Egyptian newspaper Al-Ahram al-Arabi, which put excerpts from his interview on its website on Thursday.
He said Qatar provides "weapons and money to terrorists to repeat the scenario of Libya," where Muammar Qaddafi’s regime was toppled in a bloody revolt last year.
Assad also criticized Turkey, a former close ally, saying Ankara was unconcerned "about the interests of its people, focusing solely on its ambitions that include 'the new Ottoman empire'."