Some Syrian rebels fear U.S. strikes will help Assad

While the main opposition coalition welcomed the strikes, several commanders contacted by Reuters said the campaign against Islamic State could backfire if it allowed forces loyal to Assad to fill the vacuum on the ground.

"There is a fear that the regime will exploit a military vacuum in areas of Islamic State control to realize military advances," a commander in western Syria said via online message. "This will make the alliance appear to the people as the savior of the Assad regime."

The commander, who is linked to the main rebel Free Syrian Army, said that if pro-Assad forces ended up benefiting from the strikes it could even increase the popularity of Islamic State, a hardline Sunni Muslim force which has captured tracts of land in Syria and Iraq.

The rebels themselves are not alone in worrying about the possibility that strikes against Islamic State could help Assad. France, which has struck Islamic State in Iraq but has not done so in Syria, expressed similar reservations. If there are air strikes on Islamic State "the government of Bashar cannot be left to take over," French Foreign Minister Laurent Fabius said on Monday before the strikes began. "The choice cannot be terrorists or dictators." The targets of the air and missile strikes included Raqqa city in northern Syria, Islamic State's main Syria stronghold.

Part of U.S. President Barack Obama's strategy against Islamic State depends on equipping and training Syria's more moderate rebels to strengthen the opposition as the best counterweight to the extremists.

But the rebels, who have found themselves fighting both the Islamic State and Syrian government forces, have said they urgently need heavy weaponry rather than the training and the type of light arms they expect to get.

A grouping in northern Syria of "Free Syrian Army" (FSA) fighters - as the troops affiliated with the main Western-backed opposition coalition are called - said that while it supported attacks on Islamic State, the full solution meant striking at Assad, "the source of terrorism."

"The regime plans to exploit the current situation, by taking the opportunity to massacre civilians and blame their deaths on coalition air strikes," the statement from colonel Hassan Hamadi said.

Hamadi, speaking on behalf of a grouping of northern rebel forces known as the FSA Fifth Legion, urged coordination between the FSA and U.S.-led forces to avoid civilian casualties and to allow rebels to secure areas vacated by Islamic State.

Rebel commanders also voiced concerns that the overnight raids in the northern Idlib and Aleppo provinces, which have targeted al Qaeda-linked Nusra Front, could target other Islamist brigades which have fought against Assad forces.

"It is worrying that the declared target is (Islamic State) but that the actual execution targets some FSA brigades," said FSA commander Ahmad Rahal, referring to some Islamist groups fighting alongside the FSA.

In a statement earlier on Tuesday, Syria's Western-backed political opposition welcomed the raids, saying they would bolster its struggle against Assad.

"This will make us stronger in the fight against Assad ... The campaign should continue until the Islamic State is completely eradicated from Syrian lands," Monzer Akbik, special envoy to the president of Syria's opposition National Coalition, told Reuters.

(Additional reporting by Tom Perry in Beirut and John Irish in Paris, Writing by Sylvia Westall; Editing by Sophie Walker, Janet McBride and Peter Graff)