UN chief Ban Ki-moon said on Wednesday that Syria's crisis was extremely dangerous and had 'massive repercussions' for the world, as more fighting erupted, with two Damascus suburbs coming under heavy tank bombardment.

'We do not know how events will unfold. But we do know that we all have a responsibility to work for a resolution of this profound and extremely dangerous crisis,' Ban said in a speech in the Indonesian capital Jakarta.

The crisis has potentially massive repercussions for the region and the world, he said.

On Wednesday, two large Damascus suburbs came under tank bombardment following renewed Free Syrian Army attacks on forces loyal to President Bashar al-Assad, opposition activists said.

Artillery and anti-aircraft gun barrages hit the suburbs of Harasta and Irbin, retaken from rebels by Assad's forces two months ago, and army helicopters were heard flying over the area on the eastern edge of the capital, the activists said.

Assad's forces reasserted their control over Damascus suburbs in January after days of tank and artillery shelling that beat back rebels and reduced street protests against the 42-year rule of Assad and his father, the late President Hafez al-Assad.

The suburbs are a linked series of towns inhabited mostly by members of Syria's Sunni Muslim majority, grown increasingly resentful at the domination of the Assads, who belong to the minority Alawite sect, an offshoot of Islam.

The Damascus assault and rebel fighters' flight on Tuesday from the eastern city of Deir al-Zor marked the latest setbacks for the armed opposition, which also faced accusations of torture and brutality from a leading human rights body.

But as Assad made advances on the ground, he appeared to lose ground on the diplomatic front, with key-ally Moscow adopting a new, sharper tone after months of publicly standing by his government.

'We believe the Syrian leadership reacted wrongly to the first appearance of peaceful protests and ... is making very many mistakes,' Foreign Minister Sergei Lavrov told Russian radio station Kommersant-FM.

'This, unfortunately, has in many ways led the conflict to reach such a severe stage.'

Lavrov also spoke of a 'future transition' period for Syria, but continued to reject calls from most Western and Arab states for Assad to resign, saying this was 'unrealistic'.

It was not immediately clear if the change in tone would translate into a tangible difference in the way international powers, hitherto divided on Syria, might deal with the crisis.

'The change in the Russian position is one of tone, not of substance. Moscow still sees its support of Assad as part of a regional game, but it is losing the support of the Syrian people, which could backfire on it if the Syrian regime falls,' said Najati Tayyara, a prominent Syrian opposition figure.

The uprising started with non-violent demonstrations last March, but the situation deteriorated rapidly amid a ferocious army crackdown and there are now daily clashes between rebels and security forces around the country.

The United Nations says more than 8,000 people have been killed, but the toll is rising rapidly, with at least 31 men, women and children killed on Tuesday, the British-based Syrian Observatory for Human Rights reported.

Lightly armed rebel forces have been forced into retreat across the country in recent weeks, with the army using heavy weapons to chase them from towns and cities, chalking up its latest victory in Deir al-Zor.

'Tanks entered residential neighborhoods, especially in southeastern areas of Deir al-Zor. The Free Syrian Army pulled out to avoid a civilian massacre,' a statement by the Deir al-Zor Revolution Committees Union said.

After failing to hold significant stretches of land, analysts say the rebels appear to be switching to insurgency tactics, pointing to bloody car bomb attacks in two major Syrian cities at the weekend and the sabotage of a major rail link.

Car bomb attacks in the capital Damascus and second city Aleppo killed at least 30 over the weekend, while rebels also destroyed a railway bridge linking Damascus to Deraa, according to official Syrian media.

Diplomats warn the fighting could develop into a civil war pitching Assad's Alawite sect and its allies against the Sunni majority.

Assad may also be facing pressure from inside his government. Documents described as leaked from inside Syria's embattled government show it trying to dissuade the president's allies from defecting.

The government says 2,000 members of the security forces have been killed by foreign-backed 'terrorists' and denies accusations of brutality and indiscriminate violence.

In a new twist, the advocacy group Human Rights Watch said the rebels were guilty of serious crimes, citing cases of kidnapping, torture and killings in cold blood.

Washington said it would 'absolutely denounce' human rights violations by the rebels, but stressed that most of the abuse was being carried out by pro-Assad forces.

Russia has previously vetoed two Western and Arab-backed U.N resolutions condemning government violence, arguing that the actions of rebels should also be criticized.

In a fresh effort to form a united international front, France has circulated a Western-drafted statement for the divided U.N. Security Council deploring the turmoil and backing peace efforts by U.N.-Arab League envoy Kofi Annan.

Russia announced it would back the text on two conditions - that there was no ultimatum imposed on Assad and that Annan release full details of his peace plan.

Annan dispatched a team of five experts to Damascus on Monday to discuss ways of implementing the peace drive, including a mechanism to let international monitors into the country. Syria has questioned the value of such a mission.

Lavrov also dismissed media reports of Russian warships entering Russia's naval facility in the Syrian port of Tartous.

Lavrov said a Russian tanker with fuel for Russian warships involved in anti-piracy operations in the Gulf of Aden was docked at the port.-Reuters