BEIRUT (Reuters) - 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.
"I would hope that we will continue discussions and hopefully find a language that will pull everybody together for us to move forward on this critical issue," Annan said in Moscow after meeting Russia's President Vladimir Putin.
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 rebel fighters and an activist said rebels 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 rebel 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.
Syria's Information Minister Omran Zoabi denied media reports which he said "do not reflect facts on the ground", saying security forces had confronted fighters who infiltrated the capital Damascus and forced many to flee.
"What is happening is that some armed elements infiltrated Damascus and tried to make a move in one of the areas. But the security forces surrounded them and dealt with them - and are still dealing with them," he told Reuters.
"Some (fighters) have surrendered and others escaped on foot and by car and are firing randomly in the air to frighten people," Zoabi said.
Opposition activists said clashes close to the seat of government showed that rebels 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 rebels in the street have the support of families across Damascus."
One Free Syrian Army officer said thousands of rebel fighters from the opposition centres 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 rebel said Damascus was "in a state of general alert ... We are doing well but we can not 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."
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, neighbouring 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 signalled 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.
U.S. Secretary of State Hillary Clinton, interviewed by the BBC during a Middle East tour, said she hoped Moscow would open the way for a Yemen-style transition to avoid all-out civil war. Former Yemeni President Ali Abdullah Saleh stepped down in February after months of protests, in a U.S.-backed transfer of power brokered by Yemen's wealthy Gulf Arab neighbours.
The White House also warned the Syrian government that it would be held accountable for safeguarding any chemical weapons after Western and Israeli officials said Assad appeared to be shifting some from storage sites.
British Foreign Secretary William Hague, speaking after meeting Syrian refugees in Jordan, said his visit left him in no doubt "that a Chapter Seven resolution of the United Nations Security Council is required to mandate the implementation of Kofi Annan's peace (plan)".
Chapter Seven allows the 15-member council to authorise actions ranging from diplomatic and economic sanctions to military intervention. U.S. officials have said they are talking about sanctions on Syria, not military intervention.
"We will continue to argue for it over the course of the coming hours," he told a news conference in Amman.
What began as a protest movement in Syria, inspired by demonstrations in other Arab countries, has become an armed insurgency fighting against Assad's crackdown. The International Committee for the Red Cross now classifies the conflict as a civil war.
Additional reporting by Khaled Yacoub Oweis in Amman, Oliver Holmes in Beirut, Jonathon Burch in Ankara and Jeffrey Heller in Jerusalem; writing by Dominic Evans, editing by Philippa Fletcher