US Secretary of State Hillary Clinton on Friday joined a growing international chorus of concern that Syrian President Bashar al-Assad's crackdown on pro-democracy protests may spark a civil war.

Echoing fears voiced in Russia and Turkey, Clinton warned that "a very determined, well-armed and eventually well-financed opposition," led or influenced by Syrian army defectors, may end up fighting the Assad regime.

Analysts have raised the specter of a civil war spilling over Syria's borders into neighboring countries like Lebanon, Iraq, Jordan and Israel.

In her interview with the US network NBC, Clinton stressed that the United States prefers a peaceful protest movement against Assad amid US concerns that acts of opposition violence can allow Assad to justify his crackdown.

In a number of network interviews, Clinton conceded anew how little influence the United States has over events in Syria as she backed initiatives by Turkey and the Arab League to press Assad into ending the violence.

"I think there could be a civil war with a very determined and well-armed and eventually well-financed opposition that is, if not directed by, certainly influenced by defectors from the army," Clinton told NBC television.

"We're already seeing that, something that we hate to see because we are in favor of a peaceful… protest and non-violent opposition," according to the chief US diplomat who is on a tour of Asia.

In Moscow, Russian Foreign Minister Sergei Lavrov warned Thursday that an attack like the one Wednesday on a Syrian intelligence base by army defectors could lead Syria into a civil war.

The rebel Free Syrian Army, which claimed responsibility for the raid, boasts thousands of soldiers who defected from the army to join the eight-month revolt against Assad.

Clinton's deputy spokesman Mark Toner disagreed with the Russian assessment as he said that violent acts by the opposition play into the hands of a regime that claims the protest movement is led by thugs and "terrorists."

Clinton also sought to put the blame for the violence squarely on the regime. "The way the Assad regime has responded has provoked people into taking up arms against them," she said.

In her interview with NBC, Clinton sought to dispel any idea that Washington would seek UN support for some sort of military action in Syria resembling the no-fly zone that helped Libyan rebels overthrow Moamer Kadhafi in Libya.

"There is no appetite for that kind of action vis-a-vis Syria," Clinton told the network. "Libya was a unique situation. And every place is, I think. But its not applicable to Syria."

In an interview with the US network ABC, Clinton welcomed initiatives by the 22-member Arab League and neighboring Turkey.

"We recognized early on that we were not the voice most likely to be heeded by the Syrians," Clinton said, adding that the United States had very little trade and other links with Damascus.

"And so what we've encouraged, in addition to our statements, is a growing chorus that now consists of the Arab League and Turkey that cannot be ignored by Syria," she said.

In Rabat, Arab League leaders Wednesday gave Assad three days to halt his "bloody repression" of anti-regime protests the UN says has killed more than 3,500 people, or risk sanctions.

In Ankara, Turkish Foreign Minister Ahmet Davutoglu said "it is now the right time to stop this (Syrian) massacre, and therefore the Arab initiative is important."

He added: "If it is not successful of course there is always a risk of civil war or high level tension in Syria."

Turkey on Tuesday ratcheted up the economic and political pressure on Syria, saying it would halt joint oil exploration with Damascus and threatening to stop electricity supplies over the regime's bloody crackdown on demonstrators.