BEIRUT (Reuters) - The Syrian army fired rockets into and reinforced its positions around two Damascus suburbs on Friday, resident opposition activists said, as it battles to isolate the centre of the capital from recent rebel offensives.

Opposition activists said forces loyal to President Bashar al-Assad had sent troops backed up by tanks to Moadamiah and Daraya, suburbs to the south west of the city that are close to a large military airport.

If rebels can hold those two districts they will have a continuous arc of control from the northeast to southwest of the capital, cornering Damascus against the Qasioun mountain range.

"Fifteen tanks arrived at the edge of Moadamiah this morning after and a night of very heavy artillery shelling and rocket fire into the suburbs," said an activist who asked not to be named. "They are trying to storm the area," he added.

The Syrian Observatory for Human Rights, an opposition-linked monitor, said that troops were massing on the outskirts of the areas in preparation for an advance.

Reuters could not independently confirm the reports due to severe government restrictions on media access.

Assad has been struggling to put down the 20-month-old revolt against his rule, which began as peaceful protests but has turned into a civil war that has spread to most of the country. Opposition supporters say 40,000 people have died.

Rebels have made advances of the outskirts of the capital and taken several military bases around the country in recent weeks but Assad's superior military power has leveled opposition-held areas with aerial and artillery strikes.

(Reporting by Oliver Holmes; Editing by John Stonestreet)