U.S. defense officials are accusing Iran of increased meddling in Syria, including efforts to bolster its flagging armed forces, who are weary after 18 months of fighting.

Defense Secretary Leon Panetta said Iran is building and training a militia to help President Bashar Assad's regime battle the rebel fighters trying to topple him.

The Iranian efforts, said Panetta, will only add to the killing going on in the country and "bolster a regime that we think ultimately is going to come down."

The fighting _ including alleged massacres by the regime _ has led to the deaths of more than 20,000 people, according to activists.

Sitting alongside Panetta at a Pentagon news conference Tuesday, Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the militia, which is generally made up of Syrian Shiite forces, is being used to take the pressure off the Syrian regime forces.

"Any army would be taxed with that kind of pace," Dempsey said. "They are having resupply problems, they are having morale problems, they are having the kind of wear and tear that would come of being in a fight for as long as they have."

Dempsey also said that it appears Syrian rebels were able to shoot down a Syrian warplane but said he has seen no indication that they are armed with heavy weapons or surface-to-air missiles, at least not yet.

He says the MiG fighter could have been shot down with small arms fire. Syria has blamed the crash on a technical malfunction, but Dempsey said the cause "didn't appear to be mechanical."

Dempsey and Panetta voiced concerns about Iran's growing presence in Syria even as President Bashar Assad's regime steps up its aerial attacks against the rebel forces. Fierce fighting and attacks from warplanes and helicopter gunships have pushed the opposition forces back on key fronts, such as Aleppo.

Asked about military options for intervention in Syria, Dempsey said the U.S. has been in discussions with Jordan and Turkey about the possible need for a safe zone because the two countries neighboring Syria are seeing an influx of refugees fleeing the fighting.

"And with a safe haven would probably come some form of no-fly zone, but we're not planning anything unilaterally," Dempsey said.

A no-fly zone would be a militarily enforced area in which outside nations would prohibit Syrian warplanes from flying and attacking its own people.

Panetta repeated assertions he made during an Associated Press interview Monday, saying that right now, creating a no-fly zone in the region "is not a front-burner issue" for the U.S. Instead, he said, the U.S. is focusing on providing humanitarian and nonlethal assistance and on ensuring that the chemical and biological weapons in Syria are secure.

The Obama administration has insisted repeatedly that Assad must go and that diplomatic and economic pressures, including sanctions, must be given time to work. Defense officials, including Panetta, have warned that putting a no-fly zone in place would be difficult because of the Syrian regime's relatively modern and plentiful air defense systems.

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Associated Press writer Pauline Jelinek contributed to this report.

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Lolita C. Baldor can be followed on Twitter: http://twitter.com/lbaldor