U.S. Defense Secretary Leon Panetta has suggested the Pentagon is not seriously considering a no-fly zone in Syria.
His comments in Washington come amid reports of a rising number of Syrian government air attacks on rebel forces.
Panetta said his focus is on ensuring that Syria's chemical and biological weapons sites are secure.
Panetta also accused Iran of playing a greater role in the Syrian conflict, including efforts to train a militia to fight with Assad's forces.
"We are seeing a growing presence by Iran and that is of deep concern to us, that that's taking place. We do not think that Iran ought to be playing that role at this moment in time, that it's dangerous, that's adding to the killing that's going on in Syria, and that it tries to bolster a regime that we think, ultimately, is going to come down -- but all it's going to wind up doing, frankly, is to prolong the misery of the Syrian people," Panetta told reporters at the Pentagon.
U.S. Chairman of the Joint Chiefs of Staff General Martin Dempsey said the militia is being used to take pressure off Syrian regime forces.
"The Syrian army has been fighting now for about 18 months or so and any army would be taxed with that kind of pace. [With] sanctions and other pressures, they are having resupply problems, they are having morale problems, they are having the kind of wear and tear that would come with being in a fight for as long as they have. And I actually think that's why Iran is stepping in to form this militia -- to take some of the pressure off of the Syrian military," Dempsey said.
Iran has backed Assad in his efforts to crush the rebellion inside Syria and on August 14 urged Muslim states to show greater unity ahead of a summit of Muslim leaders expected to focus on Syria.
The summit of the Organization of Islamic Cooperation is expected to consider whether to suspend Syria's membership in the 57-country Muslim bloc.
Assad's former prime minister Riyad Hijab, who defected to the opposition this month, told a news conference on August 14 that Assad controls less than a third of Syria and that his regime was collapsing.
Hijab was appointed prime minister in June. He denied Syrian government claims that he had been sacked.
Meanwhile, Panetta also said Washington does not believe Israel has made a decision on whether to attack Iran over its nuclear program.
Panetta said it was important that military action be the "last resort," adding there is still time for sanctions and diplomatic pressure to work.
Panetta's comments on August 14 at the Pentagon come amid comments by Israeli officials in recent days about a possible strike.
Israel's envoy to Washington, Michael Oren, said on August 13 in a CNN interview that the window of time before the need to resort to military action was "small and the window is getting smaller."
Based on AP and Reuters reporting