DAMASCUS: At least 30 civilians were reported killed Thursday in a massive explosion in northeast Syria as state media blamed a helicopter gunship crash on an accident but monitors said rebels shot it down.

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 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 petrol station is the only one that is still open to customers in the area, and it was packed,” a media activist who identified himself as Abu Muawiya told AFP via Skype. “It was hit by a fighter jet.

“The only reason why it would strike the petrol station with a jet is to kill the highest number of people possible,” he charged. It was impossible to verify the claim.

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 40 kilometres (25 miles) south of the border post.

The rebel Free Syrian Army battled (FSA) government troops in Raqa city, the Syrian Observatory for Human Rights said.

“Initial reports indicate that several troops were killed or injured,” it added.

The military helicopter that went down outside Damascus crashed after an accident with a civilian aircraft, state television 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.

It cited air traffic controllers at Damascus airport and Syrian Air as saying the airliner landed safely.

Earlier, the Observatory reported the helicopter was downed by rebels following a series of explosions in the restive town of Douma, northeast of the capital.

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 of them civilians.

Fierce clashes raged in rebel-held Talbisseh and Rastan in the central province of Homs, it said.

Amateur videos posted on YouTube by Talbisseh-based activists showed civilians, including children, wounded in the shelling. The Observatory said at least 90 people were killed nationwide on Thursday, including the 30 in Raqa.

On the political front, diplomats from over 60 nations and the Arab League met in the Netherlands and urged the UN Security Council to choke off the resources President Bashar al-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 has already held three meetings at ministerial level in Tunis, Istanbul and Paris. Another is planned in Morocco in October and another later in Italy.

The international community has struggled to find common ground on ways to halt the bloodshed, with Russia and China vetoing three UN Security Council resolutions condemning or threatening sanctions against the Assad regime.