At least 30 civilians were killed and dozens more wounded Thursday in an explosion at a petrol station in northeastern Syria, a human rights group said, with activists saying 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," the director of the Syrian Observatory for Human Rights, 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 that the petrol station in the village of Ain Issa, in Raqa province, was hit by a fighter jet.

"The petrol station is the only one that is still open to customers in the area, and it was packed," a media activist in the province told AFP via Skype. "It was hit by a fighter jet."

The activist, who identified himself as Abu Muawiya, accused the army of aiming to kill the highest number of civilians possible.

"The only reason why it would strike the petrol station with a jet is to kill the highest number of people possible," said Abu Muawiya.

It was impossible to verify the claim.

The deaths came a day after Syrian rebels seized control of a border crossing between Raqa province and Turkey in heavy clashes with troops loyal to President Bashar al-Assad.

Thursday's blast took place some 40 kilometres (25 miles) south of the Tal al-Abyad border post. Ain Issa lies on the main highway between Raqa city, the border post and on to the Turkish city of Sanliurfa.

"The army hit the petrol station because it wants to destroy any popular sympathy people might have towards the (rebel) Free Syrian Army," Abu Muawiya said.