ABOARD A US MILITARY AIRCRAFT: US Defense Secretary Leon Panetta said Thursday that any military action in Syria would need backing from the United Nations, but called recent violence “intolerable.”

Asked if he could foresee a scenario in which the United States would back military intervention even without UN authorisation, Panetta said: “No, I cannot envision that.”

Panetta said his duty as Pentagon chief was “to make sure that when we deploy our men and women in uniform and put them at risk, that we not only know what the mission is but we have the kind of support that we need in order to accomplish that mission.”

But he said the situation in Syria was “intolerable,” and that the United States was not ruling out any course of action.

“It’s always important for the United States of America to protect every possible option for taking action in the future,” Panetta told reporters aboard his plane bound for Singapore.

Panetta’s comments came a day after the US ambassador to the United Nations, Susan Rice, warned of possible US military intervention if Russia refuses to drop its opposition to tough sanctions against Damascus.

“I think we may be beginning to see the wheels coming off of this bus,” Rice said.

Rice spoke of three possible scenarios: Syria could implement a UN-brokered peace deal, the Security Council could ratchet up the pressure on Damascus or, failing that, outside powers could be forced to launch a military action.

But Rice appeared to back away from that stance on Thursday and did not repeat her statement.

After Panetta's briefing with reporters, his press secretary, George Little, said comments by the Pentagon chief and the UN ambassador were not “mutually exclusive.””He said very clearly that we're preserving all of our options. He's not taking any option off the table or suggesting that's administration policy,” Little said.

Panetta opposes unilateral military action and wants to see more diplomatic and economic pressure against Syrian President Bashar al-Assad's regime, Little said.

“That doesn’t mean the secretary’s comments are at odds with what she said,” the spokesman said.

President Barack Obama’s administration has expressed growing frustration with Russia over its opposition to sanctions against Assad.

Russia is Syria’s last main international ally, and has defended Assad in UN Security Council debates.

“This is an intolerable situation,” Panetta said. “We cannot be satisfied with what's going on. And the international community has got to take further steps to make sure that Assad steps down.”

Accounts from UN observers describing the regime’s killing of civilians has sparked growing international outrage and debate over whether outside powers should launch military operations.

Panetta said the US administration was worried about the conflict drawing in other countries, citing Iran’s support for Assad, as well as what might follow what he called the inevitable fall of the regime.

“I think one of the concerns is that there are countries like Iran and others that are already involved in trying to assist Assad; that there are other groups both good and bad that are engaged there as well right now,” he said. “The longer this goes on, I think the greater threat that the situation is going to get worse in terms of what happens ultimately when Assad does step down.”

Panetta, who visited US Pacific Command in Hawaii earlier Thursday, spoke at the start of a nine-day tour of Asia that will include visits to Singapore, Vietnam and India.