WASHINGTON — Senator John McCain renewed calls Monday for US military action against Syria's regime and said President Bashar al-Assad's forces are increasingly deploying attack helicopters against rebels.

Regime infantry and snipers have given way to tanks, artillery and attack helicopters as Assad seeks to crush a 15-month uprising, McCain said in a speech at a Washington think tank, the right-leaning American Enterprise Institute.

"We are now seeing a rapid increase in Assad's use of helicopter gunships," he said.

"Whereas his forces once sought to clear and hold ground, they now appear to be under orders just to kill anyone and everyone they deem a threat.

"There's every reason to believe that Assad will continue to escalate the violence, more massacres, more use of helicopters, and perhaps worse weapons after that," he said.

The US senator said Russia and Iran continued to supply Syria with arms, citing allegations from Secretary of State Hillary Clinton that Moscow was sending helicopter gunships to the regime.

"There are reports of Iranian operatives on the ground in Syria to help Assad with the killing, while Russia apparently continues to ship heavy weapons, including, as Secretary Clinton has stated, the very helicopter gunships that Assad is currently using to strafe and bomb civilians," he said.

He added that it was irrelevant whether the helicopters were new or older aircraft undergoing repairs.

McCain, who was defeated in 2008 by Barack Obama in his bid for the White House, accused the US president of failing to exercise leadership in the crisis and said Washington should rally a coalition that would conduct an air war in support of the rebels.

The Republican senator from Arizona has criticized Obama over the US troop withdrawal from Iraq as well as plans for a gradual drawdown of American combat forces in Afghanistan by the end of 2014.

"What the president does not seem to realize is what President Bill Clinton came to understand in Bosnia (in the 1990s), that a diplomatic resolution in conflicts like these is not possible until the military balance of power changes on the ground," he said.

"Our lack of involvement in Syria is not preventing the militarization of the conflict or lessening the risk of sectarian violence or countering the appeal of extremist groups," he said.

Instead, the current US approach merely meant that Washington would lose the chance to exert any influence over the outcome, said McCain.

"With or without Assad, the country will develop into a full-scale civil war, with areas of ungoverned space that al-Qaeda and its allies will occupy."

He called for forsaking the UN Security Council, due to Russia's steadfast opposition to intervention, in favor of a coalition of nations backing the use of air power.

"The US action I envision would not be unilateral; it would be multilateral. We would work closely with Arab and European allies, especially Turkey and our partners in the Gulf," he said.

He ruled out ground forces and argued in favor of an approach similar to last year's NATO-led war in Libya.

"As in Libya, there would be no boots on the ground, and we would only intervene at the request of legitimate representatives of the Syrian people."

An allied air war would carve out "safe areas in Syria" where humanitarian assistance could be staged and opposition forces could organize, he said.