Rebel troops hit youth offices of Syria's ruling Baath party on Thursday, a day after a spectacular raid on a military intelligence base outside Damascus, a human rights group said.
There was no immediate word on any casualties from the attack in Idlib province in northwest Syria, close to the Turkish border.
"A group of dissident troops attacked regime youth offices, where security agents were meeting, with rocket-propelled grenades and clashes broke out," the Britain-based Syrian Observatory for Human Rights said.
The attack came hot on the heels of a raid on an air force intelligence base in Harasta, outside the capital, on Wednesday by fighters of the Free Syrian Army, a rebel group formed by army deserters that has inflicted mounting losses on the regular army in recent months.
The rebel group announced on Wednesday it was forming a temporary military council to "bring down the current regime, protect Syrian civilians from its oppression, protect private and public property and prevent chaos and acts of revenge when it falls."
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In Washington, State Department spokesman Mark Toner said it was "not surprising" that the opposition was resorting to violence.
"We don't condone it in any way, shape or form but... it's the brutal tactics of Assad and his regime in dealing with what began as a non-violent movement (that) is now taking Syria down a very dangerous path," he said.
"We think that this kind of violence... it really plays into Assad's and his regime's hands when this becomes violent," the spokesman warned.
In Moscow, Foreign Minister Sergei Lavrov said the world community should call on all sides in Syria including the opposition to stop violence.
"There are more and more weapons that are being smuggled in from neighbouring countries," Lavrov said.
"Today I saw a television report about some new so-called rebel Free Syrian Army organising an attack on the government building, on the building belonging to Syria's armed forces," he told reporters.
"This was quite similar to a true civil war," he warned.