BEIRUT (Reuters) -- Clashes in Damascus between state forces and rebels, described by residents as the heaviest fighting to reach the capital since the start of the 15-month uprising against President Bashar Assad, spread to a second district on Sunday, activists said.

Activists in the capital said the Syrian army had closed down the road to the airport and was trying to surround fighters in southern neighborhoods such as al-Tadamon and Hajar al-Aswad in an attempt to crush unrest inside Damascus.

The fighting spread to al-Lawan, a neighborhood on the southwestern outskirts of the capital.

"There are hundreds of fighters in Damascus right now, we'll see what happens," said an activist in the capital reached by Skype, who asked not to be named. "If the regime is able to crush the fighters in Tadamon the clashes should stop, but if not they may spread further."