A Syrian brigadier-general and several other defected military officers were among 1,280 Syrians to have fled from Syria to Turkey overnight, a Turkish official said on Tuesday.
The official said the latest defections brought the number of Syrian generals sheltering in Turkey to 18, including a retired general. An undisclosed number of officers also defected with their families, totalling 68 people including family members.
A total of 1,280 refugees crossed over into Turkey's southern Hatay province overnight, bringing the number of Syrian refugees in Turkey to 42,680, the official said.
Meanwhile, troops blasted Damascus neighbourhoods with helicopter gunships and tank fire on Tuesday, witnesses said, after rebels announced an escalation of their battle for control of the capital.
Russia, meanwhile, slammed as "blackmail" Western pressure for a UN Security Council resolution against Syria's regime, and a top defector warned that Assad would not hesitate to use chemical weapons against his own people.
Fighting between President Bashar Al Assad's forces and rebels of the Free Syrian Army has raged in Damascus since Sunday, with some activists saying it marked a "turning point" in the 16-month revolt against the regime.
The FSA announced late on Monday the launch of a full-scale offensive against regime troops, dubbed "the Damascus volcano and earthquakes of Syria."
Hours later, the Britain-based Syrian Observatory for Human Rights said regime helicopters fired into Qaboon district of Damascus, while clashes broke out in the capital's Al Midan and Al Hajar Al Aswad neighbourhoods.
"Regime forces used helicopters equipped with heavy machineguns to strike the district of Qaboon," the Observatory said.
An activist in the capital told AFP that FSA fighters had blocked an attempt by the army to storm Qaboon.
"The army tried to raid the neighbourhood yesterday but the FSA stopped them," said the activist, who calls himself Omar.
He said the army, backed by tanks and low-flying helicopter gunships, randomly pounded the neighbourhood with mortars and heavy machinegun fire.
An activist who said he was in Al-Midan told AFP via Skype that the army was shelling the neighbourhood "hysterically."
"The collapsing regime has gone mad," said the man, who identified himself as Abu Musab.
"The army has tried to storm the district, but the Free Syrian Army has stopped them. So they have intensified their shelling. They are shelling everything," he said, adding that the Ghazwat Badr mosque had been destroyed.
AFP could not independently verify Abu Musab's account.
Witnesses also reported heavy machinegun fire in Sabaa Bahrat Square in the heart of Damascus, as well as in Baghdad Street, a main road near the square.
The FSA said its operation, launched at 1700 GMT on Monday, was "in response to massacres and barbaric crimes" committed by Assad's regime.
A statement by the FSA's central-Homs Joint Command described the operation launched across the country as "the first strategic step towards bringing Syria into a state of complete and total civil disobedience."
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.
Meanwhile, the Observatory said 149 people were killed across Syria on Monday -- 82 civilians, 26 rebels and 41 soldiers -- adding to its toll of more than 17,000 people dead since the uprising began.
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, who became the most prominent figure to abandon Assad when he defected as Syria's ambassador to Iraqi, warned that the beleaguered president will use chemical weapons against opposition forces and may have already deployed them.
Fares insisted that Assad'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 taken 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.
"I made it very clear we are going to vote against this resolution," Russian Ambassador Vitaly Churkin said after the latest tense talks among council ambassadors.
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.