(Reuters) - Syrian rebels said they shot down an army helicopter 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 center.

Col. 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.

Video uploaded by opposition activists showed Damascus buildings and shops that were set ablaze by what they said was rocket and artillery fire. One showed a grocer sweeping out the scorched contents of his shop as smoke still rose from the back.

I couldn't sleep at all. There was shelling with artillery and helicopter gunships from midnight until 6 in the morning. They didn't stop firing for a single minute, said a Damascus resident, contacted from Beirut by telephone.

They were using artillery, helicopter gunships and mortars. At dawn you could hear the call to prayer mixed with the sound of gunfire. Now we can hear clashes with heavy machinegun fire. Helicopters are hovering over the area.

Clashes continued in the Midan district in central Damascus and artillery and rocket fire also hit the opposition area of Tadamon, on the outskirts, residents said. Many roads in and out of the capital had been closed, they added.