DAMASCUS - Syria's military deployed armoured vehicles near central Damascus on Monday as troops battled rebels around the capital in what activists said could be a turning point in the 16-month uprising.

Meanwhile, Russia slammed as "blackmail" Western pressure to push for a United Nations Security Council resolution against Syria's regime and said it would be "unrealistic" for President Bashar al-Assad, its ally, to quit.

With battles raging between the army and rebels around Damascus for a second straight day, troops deployed armoured vehicles near the historic neighbourhood of Al-Midan.

"This is the first time that armoured and military transport vehicles are deployed in Al-Midan," Rami Abdel Rahman of the Syrian Observatory for Human Rights, the Britain-based watchdog, said in Beirut.

An activist on the ground said the army was trying to overrun Al-Midan and described the fighting as a "turning point" in the revolt against Mr Assad's autocratic regime.

The battles are "the first of their kind. You can say there is a before and after in the Syrian revolution, and the turning point was July 15," said the activist who identified himself as Abu Musab. "The army is trying to storm Al-Midan from two sides, with military vehicles. There are many injured and some killed."

Activists said the army and Free Syrian Army rebels had been locked in fierce clashes since Sunday in the southern Damascus neighbourhood of Tadamon, Kfar Sousa in the west and Jobar in the east. They said the clashes were the worst in the capital since the start of the uprising in March 2011.

"Mortar shelling resumed in the early morning," targeting Tadamon, said the Local Co-ordination Committees activist network.

The Syrian Observatory for Human Rights reported "dawn battles on the road south of Kfar Sousa, between rebel fighters and soldiers who were in a convoy passing through the area".

A resident of nearby Jaramana said the area was like a "war zone."

Activists said residents were fleeing Tadamon, with many seeking shelter in the nearby Yarmuk Palestinian refugee camp.

The pro-government Al-Watan newspaper said the army was battling "terrorist groups" and accused the gunmen of seeking to launch "the great Damascus battle".

But the opposition Syrian National Council accused the regime of transforming Damascus into "battlefields".

In Geneva, the International Committee of the Red Cross (ICRC) sounded a note of alarm, saying Syria was in a state of all-out civil war and that all sides had to respect humanitarian law or risk facing war crimes prosecutions.

"Each time there is fighting we can see conditions that can be defined as a non-international armed conflict," ICRC spokesman Alexis Heeb said, adding that "international humanitarian law applies" in such circumstances.

The latest violence comes as diplomatic pressure builds ahead of a key Security Council vote on Friday to decide if the 300-strong UN supervision mission in Syria will be renewed.

The unarmed observers are tasked with overseeing the implementation of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan, which has been flouted daily since mid-April when it went into effect.

Speaking ahead of talks with Annan, Russian Foreign Minister Sergei Lavrov accused the West of trying to "blackmail" Moscow to get its backing for possible sanctions against Syria.

"To our great regret, we are witnessing elements of blackmail," said Mr Lavrov, adding it was "unrealistic" for Moscow to back calls for Mr Assad to step down as the population supported him. "It is simply unrealistic ... he will not leave power. And this is not because we are protecting him but because there is a very significant part of the Syrian population behind him."

Mr Annan is on his way to Moscow for talks with Mr Lavrov and President Vladimir Putin while UN chief Ban Ki-moon is due in Beijing on Tuesday also for a mission to get support for tougher action on Syria.

Moscow and Beijing have twice blocked resolutions against Syria at the Security Council, which is divided over Western calls to pile new sanctions on Damascus.

Britain, the US, France, Germany and Portugal want a resolution passed this week that would threaten sanctions if Mr Assad does not pull back his main weapons.

The diplomatic moves come after Syria denied its troops carried out a massacre in the central village of Treimsa, where activists said dozens of people were slaughtered on Thursday by troops and pro-regime militiamen.

Syria has denied there was a massacre while UN observers are probing the reported killings.

On Sunday, violence across Syria killed 105 people, the Observatory said, adding to its toll of more than 17000 people killed in the country since the uprising began.

Meanwhile, Morocco declared Syria's ambassador to Rabat persona non grata and asked him to leave the country, before Damascus said the Morocco envoy in its capital was unwelcome, in a tit-for-tat move.