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.
Russia, meanwhile, slammed as "blackmail" Western pressure to push for a UN Security Council resolution against Syria's regime, as a top defector warned that President Bashar al-Assad would not hesitate to use chemical weapons against his own people.
Syria's rebels late Monday announced the launch of a full-scale attack operation.
The Free Syrian Army's central-Homs Joint Command said its operation was launched "in response to massacres and barbaric crimes" committed by the Assad regime.
The FSA, the statement said, started to conduct "attacks on all security stations and branches in the cities and the countryside, to enter into fierce clashes (with their forces) and to call on them to surrender."
The FSA called for all international roads to be cut off, "from (northern) Aleppo to (southern) Daraa and from (eastern) Deir Ezzor to (coastal) Lattakia, to cut off and seize the supply lines."
As battles raged around Damascus for a second straight day, troops deployed armoured vehicles near the historic neighbourhood of Al-Midan.
"When there is fighting in the capital for several hours, even days, and troops are unable to control the situation, that proves the regime's weakness," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP.
Online videos showed street battles in the capital, with fighters firing off rocket-propelled grenades from behind sandbags.
An activist on the ground, identifying himself as Abu Musab, said the army was trying to overrun Al-Midan and called the fighting a "turning point" in the revolt against Assad's autocratic regime.
Activists said the army and FSA rebels had also been locked in fierce clashes since Sunday in the southern Damascus neighbourhood of Tadamon, Kfar Sousa in the west and Jobar in the east.
The Observatory said at least seven people were killed, all but one of them civilians, in the Midan, Tadamun and Aishe districts, in the heaviest clashes in the capital since the March 2011 start of the uprising.
The authorities vowed on Monday they would not surrender the capital. "You will never get Damascus," read the headline in Al-Watan newspaper, which is close to the regime.
Activists said residents were fleeing Tadamon, with many seeking shelter in the nearby Yarmuk Palestinian refugee camp, as the opposition Syrian National Council accused the regime of transforming Damascus into "battlefields."
Rebel-held districts of the central city of Homs, which has been under siege for months, were also bombarded on Monday, according to the Observatory.
It said a total of at least 67 people were killed in violence across the country—32 civilians, 21 soldiers and 14 rebel fighters.
In Geneva, the International Committee of the Red Cross said Syria is in a state of all-out civil war and that all sides must respect humanitarian law or risk 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.
Nawaf Fares, the first Syrian ambassador to defect, warned Assad will use chemical weapons against opposition forces and may have already deployed them.
Fares, the most prominent politician to defect since the uprising against Assad began, insisted the president's days were numbered but warned he would be prepared "to eradicate the entire Syrian people" to remain in power.
When asked by the BBC's Frank Gardner whether that would mean the use of chemical weapons, Fares said: "I am convinced that if Bashar al-Assad's regime is further cornered by the people, he would use such weapons."
The latest violence comes as diplomatic pressure builds ahead of a key Security Council vote to decide if the 300-strong UN Supervision Mission in Syria (UNSMIS) will be renewed on Friday.
The unarmed observers are tasked with overseeing 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 was to have gone into effect.
Security Council talks on Syria virtually collapsed Monday, leaving the major powers heading for a veto showdown on a proposal to impose sanctions on Assad.
Russia will veto a western resolution linking the renewal of the UN mission with sanctions when it comes to a vote on Wednesday, its UN ambassador Vitaly Churkin said after council talks.
A rival Russian resolution just proposing to renew the UN mission would fail to get enough votes from the 15 council members to pass, US envoy Susan Rice told reporters. Russia is Assad's main ally.
Annan is in Moscow for talks with Foreign Minister Sergei Lavrov and President Vladimir Putin while UN chief Ban Ki-moon is due in Beijing on Tuesday, also on a mission to get support for tougher action on Syria.
Russia and China have twice blocked resolutions against Syria at the Security Council which is divided over Western calls to pile new sanctions on Damascus.
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.
On Sunday, violence across Syria killed 105 people, the Observatory said, adding to its toll of more than 17,000 people dead since the uprising began.