At least 30 civilians were reported killed on 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 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 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 Observatory said, adding: "Initial reports indicate that several troops were killed or injured."

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, 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.

Amateur videos posted on YouTube by Talbisseh-based activists showed civilians -- including children -- wounded in the shelling.

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.

On the political front, 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 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.

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 Moamer Kadhafi's regime was toppled in a bloody revolt last year.

Assad also criticised 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'."

Overnight, UN chief Ban Ki-moon warned that the Syrian government and rebels seemed intent on fighting to the bitter end and said the conflict would top the agenda at next week's UN General Assembly meeting.

Ban said UN-Arab League envoy Lakhdar Brahimi, who just wrapped up his first visit to Syria and its neighbours since taking up his post earlier this month, may put a plan to Assad's government after the UN talks.

Helicopter gunships pounded the Al-Hajar al-Aswad district of the capital, as the exiled opposition said a large swathe of south Damascus had been turned into a disaster area by the heavy fighting.

"Helicopter gunships are pounding civilian homes in Al-Hajar al-Aswad in south Damascus," the Syrian National Council said, adding: "Many people have been killed or injured."

The SNC also renewed its call on the international community to intervene, saying its "response to what is happening in the world's oldest capital city (Damascus) has been completely insufficient."

On Wednesday, the Syrian Revolution General Commission (SRGC), a grassroots network of anti-regime activists, also spoke of disastrous conditions in south Damascus and pleaded for help from international aid organisations.

The UN's World Food Programme appealed for greater access to the "hotspot areas" of Syria, saying it was not able to assess aid needs there.