Syrian armed forces launched a massive operation on Friday to secure a strategic military airport near Damascus ahead of possible US-led military strikes, a monitoring group said.

Troops bombarded rebel positions in Moadamiyet al-Sham, a town southwest of Damascus near Mazzeh airport, one of the country's key air bases, said the Syrian Observatory for Human Rights.

Observatory director Rami Abdel Rahman said: "The regime is trying to seize control of Moadamiyet al-Sham before a probable Western strike, because the town commands access to Mazzeh airport".

The base is guarded by the army's elite Fourth Armoured Division -- a unit led by President Bashar al-Assad's brother Maher -- which is responsible for security in and around Damascus.

The army has been unsuccessfully trying for months to take Moadamiyet al-Sham back from the rebels.

Western leaders accuse the Assad regime of unleashing chemical attacks on August 21 east and southwest of Damascus, including in Moadamiyet al-Sham, that killed hundreds.

US President Barack Obama has been trying to secure international support for a punitive strike on regime targets and will seek congressional approval for such action next week.

The Syrian government denies using chemical weapons, blaming instead rebels trying to topple the regime of using the poisonous agents.

The Britain-based Observatory, which relies on a network of activists and medics on the ground for its reports, said at least four surface-to-surface missiles fired by regime forces struck Moadamiyet al-Sham Friday.

Rebels and troops also clashed in the area as Assad's forces tried to rout the insurgents, it added.