Dozens of people, including rebels, were killed or wounded in an air strike on a service station southeast of Damascus on Wednesday, many of them horribly burned, a watchdog and activists said.

The attack took place near the town of Maliha in the partly rebel-held Eastern Ghuta region of Damascus, the Syrian Observatory for Human Rights said.

The watchdog said dozens were killed or wounded but could not immediately give a comprehensive breakdown.

"There are 12 bodies that have been found at the scene, including a number of rebels from different local battalions," Observatory director Rami Abdel Rahman told AFP by phone.

"It is not yet clear if the gas station was the target," he added.

The Local Coordination Committees, a grassroots network of activists, estimated that at least 50 people were killed and dozens of others wounded.

It said the toll was likely to rise because bodies were still being pulled from the rubble, adding that "it is extremely difficult to count the dead because most of the bodies have been immolated."

It was not immediately clear if the bomb blasts caused the storage tanks to explode, but the scene was engulfed in fire, which suggests that was the case.

"MIG warplane strikes on Eastern Ghuta! Dozens of martyrs!" a man shouted out as he and a fellow cameraman raced toward plumes of smoke to survey the damage in a gruesome video posted on YouTube.

A man stood wailing to God as he held what was left of his friend, a head and a shredded torso with a bloodied shirt still hanging on flaps of skin.

A fire extinguisher lay in the street as dozens of men rushed to dig out any survivors from beneath heaping piles of metal and steel I-beams as fires raged beside them.

One man, his face covered in blood, was helped out from the rubble.

The camera panned to show the body of another man on the hood of a car, charred from head to toe, his severed leg splayed out beside him as a fire ravaged the right side of his body.

Another man was still atop a motorcycle in the middle of the fire, his body engulfed in flames.

Some bystanders stood in shock, staring into space, surrounded by scattered burning vehicles, heaps of metal and pools of blood still wet on the pavement.

In another video, a man's leg is charred down to the bone. The camera continues to show his entire burned and mutilated body, ending at his face. The man was still conscious and coughing up blood.

The authenticity of the video could not immediately be verified.

Meanwhile 12 members of the same family, most of them children, were killed in an air raid in Moadamiyet al-Sham, a town southwest of Damascus which is partly held by rebels, the Observatory said.

Warplanes also attacked insurgent strongholds across other parts of the capital.

The Observatory gave an initial toll of 59 people killed nationwide on Wednesday.