"With regard to the no-fly zone, that is not a front-burner issue for us," Panetta told reporters at the Pentagon after Syrian rebel forces appealed for an internationally sponsored no-fly zone as added protection from escalating Assad regime airstrikes.
Joint Chiefs of Staff Chairman U.S. Army Gen. Martin Dempsey, in the same news briefing, said Jordan and Turkey might consider imposing a no-fly zone, over which Syrian military aircraft would not be permitted to fly.
"We have been in discussion with Jordanians and the Turks, and they're both interested mostly in the effects that could spill over from Syria into their countries," said Dempsey, the principal military adviser to Panetta, President Barack Obama, the National Security Council and the Homeland Security Council.
"Both [countries] have examined the possibility of a safe haven [for refugees fleeing Syrian fighting]. And with a safe haven would probably come some form of no-fly zone, but we're not planning anything unilaterally," he said.
Amman and Ankara had no immediate comment.
Secretary of State Hillary Clinton, who visited Turkey Saturday, said afterward the two countries discussed a range of steps that a Turkish official said included a no-fly zone over parts of Syria.
No decision on the zone was reached, the official said.
A no-fly zone imposed and enforced by Washington and NATO allies last year is widely credited with being crucial to helping Libyan rebels overthrow dictator Moammar Gadhafi.
Syria's air defenses are more modern and substantial than Libya's, Western military advisers say.
Panetta expressed concern about what he termed Iran's growing presence in Syria, with Iranians allegedly training and advising a Syrian militia supporting President Bashar Assad's forces.
"We do not think that Iran ought to be playing that role at this moment in time -- that it's dangerous, that it'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," Panetta said.
Dempsey said it appeared Syrian rebels for the first time shot down a Syrian jet fighter Monday, despite Damascus' insistence the jet crashed because of a technical failure.
He said he had no indication the rebels were armed with heavy weapons or surface-to-air missiles. The shoot-down had raised questions about the opposition's military capabilities in the 17-month-old conflict.
Dempsey said the warplane could have been brought down with small-arms fire.