Forces loyal to Syrian President Bashar al-Assad have used chemical weapons to dislodge rebel troops from a key military base in north-eastern Damascus, according to a key opposition activist.

Mohammed al-Doumani told Reuters that the rebels used phosphorus in rockets fired on the town of Adra, in an attack which has killed at least two people and injured several others.

"Doctors are describing the chemical weapon used as phosphorus that hits the nervous system and causes imbalance and loss of consciousness," Douhami said.

Regarding the casualties, the activist said the two dead fighters "were very close to where the rockets exploded and they died swiftly. The rest are being treated with Atropine".

A similar attack last week had killed 26 people in Aleppo; both the rebels and Assad's regime blamed each other for the attack.

Both sides have been accused of using chemical weapons, which are banned under international law, but both have repeatedly denied the accusations.

In a separate attack, the commander of the Free Syrian Army (FSA), Riad al-Assad, has sustained serious injuries, according to reports.

"Assad's attempt to kill Riad al-Assad was to punish the eastern areas, such as Al-Reqqa and Deir Ezzor, held by rebels," FSA spokesperson Louay Almokdad told al-Arabiya, adding that he is now being treated in Turkey.

He was wounded during a bomb blast in the eastern city of Deir Ezzor.