Troops have clashed with rebels in the Syrian capital Damascus for a second day in what activists say is some of the worst fighting since the country's crisis began 16 months ago.

The clashes briefly closed the motorway linking the capital with Damascus International Airport and the city's south - an unprecedented development, according to Mustafa Osso, an activist based in Syria.

"It seems there is a new strategy to bring the fighting into the centre of the capital," Mr Osso told reporters, referring to rebels who fight under the banner of the Free Syrian Army.

"The capital used to be safe. This will trouble the regime."

The UK-based Syrian Observatory for Human Rights and Mr Osso said the fighting was concentrated in the districts of Kfar Souseh, Midan and Tadamon.

An amateur video posted online showed Kfar Souseh as the sound of intense gunfire could be head in the distance. The presidential palace, on a mountain overlooking the capital, could be seen in the background. The authenticity of the video could not be independently confirmed.

There have been sporadic clashes in Damascus in recent months, although president Bashar Assad's forces remain firmly in control of the capital. Many of the Damascus suburbs, however, have risen up against the regime, prompting a ferocious response from the military in an attempt to clear out rebel fighters from the towns that ring the capital.

On Monday, activists reported government attacks in the Damascus suburb of Qatana.

Rami Abdul-Rahman, who heads the Observatory, said Monday's fighting is taking place roughly one mile from the airport highway. One Damascus resident who did not wish to be named said gunfire and sporadic explosions could be heard throughout the morning. The resident said that unlike previous clashes that occurred at night, the recent fighting took place during the day - a sign that the rebels are becoming more brazen.

On Sunday, Syria's 16-month conflict crossed an important symbolic threshold as the International Red Cross formally declared it a civil war, a status with implications for potential war crimes prosecutions.

PA