WASHINGTON: US Defence Secretary Leon Panetta says plans to set up a no-fly zone over parts of Syria are "not on the front burner," despite persistent calls from Syrian rebel forces that they need the added protection from escalating regime airstrikes.

Panetta is confident that the US could successfully enforce a no-fly zone over Syria, but doing so would require a "major, major policy decision" that has not yet been made, he told a news agency.

"We have planned for a number of contingencies that could take place and one of those possible contingencies is developing a no-fly zone. But we've also pointed out difficulties in being able to implement that," he said. "It's not on the front burner as far as I know."

Secretary of State Hillary Rodham Clinton said recently that Washington and Turkey are discussing a range of steps, including a no-fly zone over some parts of Syria. Rebel leaders have expressed frustration that the United States has limited its assistance to non-lethal aid.

The US and its NATO allies successfully enforced a no-fly zone over Libya last year, as rebels there made gains and eventually ousted dictator Muammar Gaddafi.

Syria, however, has relatively modern air defences that are far more plentiful and sophisticated than those in Libya. Syria buys its arms from Russia and is backed in its efforts to tamp down the rebels by Iran.

Syrian President Bashar Assad's military has significantly stepped up aerial attacks in recent weeks, using missile strikes to push back opposition forces in key fronts such as Aleppo. Civil war has spread across the country, and activists say more than 20,000 people have been killed since the revolt against Assad began in March 2011.

Currently, Panetta said, the US is focused on ensuring chemical and biological weapons in Syria are secure and on providing humanitarian and non-lethal assistance to the rebels.