Troops fought fierce battles with rebels in Damascus on Monday for a second straight day as Russia dismissed as "blackmail" Western pressure to back a UN Security Council resolution against Syria.
The International Committee of the Red Cross said meanwhile that Syria is in a state of all-out civil war and that all sides must 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 told AFP, adding "international humanitarian law applies" in such circumstances.
But the violence raged on Monday, with troops resuming shelling of the southern Damascus neighbourhood of Tadamon and soldiers and the Free Syrian Army (FSA) clashing in two other districts, activists said.
They said the clashes that erupted on the capital's outskirts on Sunday is the worst since the uprising against President Bashar al-Assad's autocratic regime broke out in March 2011.
"Mortar shelling resumed in the early morning," targeting Tadamon, as troops and rebels fought fiercely in Kfar Sousa in the west and Jobar in the east, said the Local Coordination 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."
"(It has) never been this intense," the Observatory's Rami Abdel Rahman told AFP.
A resident of nearby Jaramana said the area had been turned into a "war zone," keeping him awake all night. Activists reported "tanks entering Tadamon and ensuing clashes."
The pro-government Al-Watan newspaper said the army was battling "terrorist groups" who had taken up positions in outer neighbourhoods intending to launch "the great Damascus battle."
Tadamon, Kfar Sousa and Jobar are centres of anti-regime dissent which Abdel Rahman said the army was trying to reclaim but has so far failed to do as FSA fighters had locked them down.
The opposition Syrian National Council accused the regime of transforming Damascus into "battlefields" and hailed the insurgents.
"The revolution is spreading and has tightened the noose around the regime in zones where it thought it was beyond the anger of the people," said SNC spokesman Georges Sabra.
The latest violence comes as the diplomatic pressure builds ahead of a key Security Council vote on Friday to decide if a 300-strong UN observer mission to Syria will be renewed.
The UN Supervision Mission in Syria, or UNSMIS, is tasked with overseeing the implementation of a six-point peace plan brokered by envoy Kofi Annan which has been flouted daily since mid-April when it went into effect.
Speaking ahead of talks with Annan, the UN-Arab League envoy, 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 Lavrov, adding it was "unrealistic" for Moscow to back calls for Assad to step down as the population supports 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," Lavrov told reporters.
Annan is on his way to Moscow for talks with Lavrov and President Vladimir Putin while UN chief Ban Ki-moon is due in Beijing where he will meet President Jintao on a mission Tuesday 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.
"So divided that maybe Annan and Ban now have the most influence over Russia and China to get anything done," a senior UN council diplomat said.
Britain, the United States, France, Germany and Portugal want a resolution passed this week that would threaten sanctions if 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 Thursday by troops and pro-regime militia.
"What happened was not a massacre... It was a clash between regular forces and armed groups who do not believe in a peaceful solution," foreign ministry spokesman Jihad Makdissi said, insisting only five buildings were targeted.
UN observers went into Treisma on Sunday to probe the reported killings and "observed over 50 houses that were burned and/or destroyed," said UNSMIS spokeswoman Sausan Ghosheh.
"Pools of blood and brain matter were observed in a number of homes," she said, adding that the attack appears targeted at army defectors and activists."
"The number of casualties is still unclear."
Activists said more than 150 people were massacred in the village. If that number is confirmed, it would make it one of the bloodiest episodes of the uprising.
On Sunday, violence across Syria killed 105 people, the Observatory said, adding to its toll of more than 17,000 people killed in the country since the uprising began.