While the U.S. and its European allies have backed the opposition, they have been reluctant to provide those fighting Assad’s troops with weapons that could challenge the regime’s superior firepower. They fear the arms could end up in the hands of radical Islamic groups, such as the al-Qaida-affiliated Jabhat al-Nusra, that in the past year have become the most effective fighting force on the opposition’s side.

On Thursday, U.S. Defense Secretary Chuck Hagel said Washington is rethinking its opposition to arming the rebels amid growing indications that the Assad regime has used chemical weapons — something President Barack Obama has called a ‘‘red line.’’

The U.S. has said intelligence indicates Syria has used the nerve agent sarin on at least two occasions, but Obama has stressed that he needs more definitive proof before making a decision about how to respond — and whether to take military action. Damascus denies using chemical arms, and says the opposition is trying to frame it.

Obama said Thursday his administration was proceeding cautiously as it looked at options to ensure that what it does is helpful to the situation rather than making it more deadly or complex.

The Syrian crisis started with largely peaceful protests of Assad’s rule in March 2011, but shifted into an armed insurgency as opposition supporters took up weapons to fight a harsh regime crackdown on dissent.

The conflict has laid waste to cities and towns, forced more than 1 million Syrians to seek refuge in neighboring countries such as Jordan, Lebanon and Turkey, and displaced millions more inside Syria.

Syria’s state news agency said mortars fired at the Damascus International Airport struck a kerosene tank and a commercial aircraft, causing significant material damage. It said the airport was functioning normally, although many airlines no longer fly into the Syrian capital because of the conflict.

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