Beirut - Syrian rebels said they shot down an army helicopter on Tuesday as they battled government forces backed by air power and artillery in the fiercest fighting to hit Damascus since the revolt against President Bashar al-Assad erupted last year.

Rebel officers said thousands of fighters had converged on the capital, a government stronghold during 16 months of turmoil, to bring the battle from Syria's turbulent provinces into Assad's power centre.

Colonel Qassem Saadeddine, spokesman of the joint command of the Free Syrian Army inside Syria, told Reuters via Skype the rebels would intensify attacks inside Damascus and target sensitive security installations in what is called now an operation to "liberate Damascus".

"There is no going back. The Damascus battle has priority for us. We have started the operation to liberate Damascus," Saadeddine said, adding the rebels had called their operation "Damascus volcano and Syrian earthquake".

But it was not clear whether either side could deliver a decisive blow. Free Syrian Army fighters said they had killed 70 members of the security forces and pro-Assad militiamen known as shabbiha over the past 24 hours.

The encroachment of violence into the capital came as United Nations envoy Kofi Annan said he hoped the U.N. Security Council would be able to reach agreement on a Syria resolution.

The council is due to vote on Wednesday on a Western-backed resolution that threatens Syrian authorities with sanctions if they do not stop using heavy weapons in towns, but Russia has said it will block the move.