The commander of Syrian rebels in the northern province of Aleppo says his forces have launched an operation aimed at "liberating" the country's largest city from regime troops.
The statement by Col Abdul-Jabbar Mohammed Aqidi posted by activists on YouTube came as clashes in the country's commercial hub of Aleppo raged for a third day.
The Britain-based Syrian Observatory for Human Rights and Aleppo-based activist Mohammed Saeed said the fighting is concentrated in several neighbourhoods. The Observatory also reported attacks by government forces in the capital Damascus in the neighbourhoods of Mazzeh and Barzeh.
Damascus and Aleppo are both home to elites who have benefited from close ties to president Bashar Assad's regime, as well as merchant classes and minority groups who worry their status will suffer if Assad falls.
The attack on Aleppo, Syria's commercial hub that has been a bedrock of support for president Bashar Assad, was a sign of the rebels' growing confidence and capabilities days after they killed four members of Assad's inner circle in a Damascus bombing.
"Right now, Assad's inner circle has been dismantled and Assad has lost his balance," brigadier general Abdul Kareem al-Ahmad of the rebel Free Syrian Army said at a meeting in Turkey. "This war is now being waged in the heart of Syria in Damascus."
The killing of senior regime figures, a series of high-level military defections, and the capture of several border crossings have given the rebel side unmistakable momentum over the past week and put the regime on the defensive. After struggling for nearly a week to put down a rebel challenge inside the capital, regime forces appeared close to regaining control of Damascus.
The escalating bloodshed and increasing chaos is threatening to spill across borders into a larger regional conflagration. It has put Syria's neighbours, particularly Israel, on edge.
Israeli prime minister Benjamin Netanyahu said in an interview with Fox TV that his "principle concern" is the political chaos that might ensue if Assad falls and the Syrian- and Iranian-backed Lebanese militant group Hezbollah gains access to Syria's stockpile of chemical weapons, rockets and missiles.
He said Israel has not considered specifically trying to cross the border and seize the weapons. "There are other possibilities," he said without elaborating. "We'll have to consider our actions. Do I seek action? No. Do I preclude action? No."