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, and said it would be "unrealistic" for its ally President Bashar al-Assad to quit.
"Al-Midan and Tadamon are out of the army's control," said Ahmed al-Khatib, spokesman for the Free Syrian Army's (FSA) military council in Damascus.
"The army has no presence inside either of these neighbourhoods any more, though they are shelling from the outside, and clashes on the edges of the neighbourhoods continue."
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.
"Security forces, backed by the army, have for the past 48 hours been attacking the terrorist groups as they try to pull back to districts on the outskirts," the paper said.
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, 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.
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 (UNSMIS) will be renewed.
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.
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 Lavrov, adding that it was "unrealistic" for Moscow to back calls for Assad to step down as the population supports him.
Annan is in Moscow for talks with 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.
Meanwhile, Morocco declared Syria's ambassador to Rabat persona non grata and asked him to leave, before Damascus said the Moroccan envoy there was unwelcome, in a tit-for-tat move.
In Geneva, a top UN relief official said the Damascus regime was guilty of "tremendous political obstruction" that was preventing aid reaching an estimated 850,000 needy people in Syria.
In Brussels, meanwhile, EU diplomats said that European Union foreign ministers plan to adopt a 17th round of targeted sanctions against Assad's regime at talks next week.