Syrian revolution fighters 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.

Revolution 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.

Colonel Qassem Saadeddine, spokesman of the joint command of the Free Syrian Army inside Syria, told Reuters via Skype the fighters 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 fighters 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.

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.

Two fighters and an activist said they shot down an army helicopter in the Qaboun district in north-east Damascus. "Helicopters are flying at low altitude. It's easy to target them using anti-aircraft weapons," a senior revelution officer said.

Israel's army intelligence chief said Assad's control of Damascus was slipping and he had redeployed troops from areas near the Israeli frontline to bolster forces around the city.

"Assad has moved many of his forces that were in the Golan Heights to the conflict areas," Major-General Aviv Kochavi said. "He's not afraid of Israel at this point, but primarily wants to bolster his forces around Damascus.

"Draining the regime"

Opposition activists said clashes close to the seat of government showed that fighters were chipping away at state power in a capital once seen as Assad's impenetrable stronghold.

"When you turn your guns against the heart of Damascus, on Midan, you have lost the city," said Damascus-based activist Imad Moaz. "The revolution fighters in the street have the support of families across Damascus."

One Free Syrian Army officer said thousands of fighters from the opposition centers of Idlib, Raqqa, Hama and Homs had moved to Damascus, but lacked firepower to deliver a decisive blow and were seeking instead to "drain the regime".

Another senior revolution fighter said Damascus was "in a state of general alert ... We are doing well but we cannot announce anything yet. We cannot talk about seizing an area because the regime will then destroy it completely."

"It is too soon to talk about toppling him now."

Refugees flee

While fighting raged in Damascus, a Turkish official said a Syrian brigadier general and several other military defectors were among 1,280 Syrians to have fled to Turkey overnight.

They were the latest in a steady stream of officers to join the revolt, and follow the defection of Syria's ambassador to Iraq last week and the escape from Syria by Manaf Tlas, a member of Assad's inner circle.

Tlas has not spoken publicly since defecting but French President Francois Hollande gave the first official confirmation on Tuesday that he was in France.

Underlining the depth of the crisis, neighboring Iraq called on its citizens - many of whom had fled to escape Iraq's own sectarian bloodshed - to leave Syria.

Clashes and shelling in opposition areas continued across the country and the British-based Syrian Observatory for Human Rights, which has a network of activists across the country, said more than 150 people were killed on Monday.

Activist accounts are hard to verify because the government restricts access to international media.

With violence rising, the West wants Moscow to drop its support for Assad. Along with China, Moscow has vetoed action against the president at the U.N. Security Council. But before talks with Annan, Foreign Minister Sergei Lavrov signaled no change in Moscow's position.

Lavrov said Western efforts to pass a Security Council resolution, which would extend a U.N. monitoring mission in Syria and also include a threat of sanctions, contained "elements of blackmail". He called for support for Moscow's rival text, which does not call for sanctions.

"If our partners decide to block our resolution no matter what, then the U.N. mission will not have a mandate and will have to leave Syria. That would be a pity," he said.

The small, unarmed U.N. monitoring mission of about 300 is the only international military presence in Syria. It was brought in as part of a peace plan backed by Annan, but suspended due to rising violence in Syria. Activists say more than 17,000 people have died.

PHOTO CAPTION

Amateur video out of Syria appears to show protesters blocking a major highway in Damascus.

Reuters