At least 11 people have been killed with many more wounded after a car bomb exploded at a petrol station in the Syrian capital Damascus.

The petrol station was packed with people queueing for fuel that has become increasingly scarce during the country's 21-month-long civil war, according to opposition activists.

The bombing took place in the Barzeh al-Balad district, whose residents include a mix of majority Sunni Muslims and several other religious and ethnic minorities.

The opposition Revolution Leadership Council in Damascus said in a statement: "A booby-trapped car at the site killed several people and injured dozens."

There was no immediate indication of who may have been responsible.

An activist who lives in the area said he saw ambulances loading burnt bodies and wounded people with severe burns before he was ordered away by security forces surrounding the station.

"The station is usually packed even when it has no fuel. There are lots of people who sleep there overnight, waiting for early morning fuel consignments," said the activist, who did not want to be named.

He said brawls have increased lately at the petrol station between militia loyal to President Bashar al-Assad and ordinary citizens waiting for fuel, but it was not known who carried out the attack.

Air base battle

Rebels are battling to seize an air base in northern Syria, part of a campaign to limit the air power that has given President Assad the upper hand.

More than 60,000 people have been killed in the 21-month uprising and civil war, the United Nations said this week.

After dramatic advances over the second half of 2012, the rebels now hold wide swathes of territory in the north and east.

However, they are limited in exerting control because they cannot protect towns and villages from Mr Assad's helicopters and jets.

Hundreds of fighters from rebel groups were attempting to storm the Taftanaz air base, near the northern highway that links Syria's two main cities, Aleppo and the capital Damascus.

Rebels have been besieging air bases across the north in recent weeks.

A rebel fighter speaking from near the Taftanaz base overnight said the base's main sections were still in loyalist hands but insurgents had managed to infiltrate and destroy a helicopter and a fighter jet on the ground.

The northern rebel Idlib Coordination Committee said the rebels had detonated a car bomb inside the base.

The government's SANA news agency said the base had not fallen and that the military had "strongly confronted an attempt by the terrorists to attack the airport from several axes, inflicting heavy losses among them and destroying their weapons and munitions".

Rami Abdulrahman, head of the opposition-aligned Syrian Observatory for Human Rights which monitors the conflict from Britain, said as many as 800 fighters were involved in the assault.