The United States said Thursday there was no truth to reports it was ready, along with Britain, to offer Syrian President Bashar al-Assad safe passage to travel to Geneva to meet opposition leaders.

Washington said it was not its role to decide Assad's fate, but outside pressure nevertheless built on his regime, following a report the CIA was now vetting the flow of arms provided by Middle Eastern powers to Syrian rebels.

Officials also welcomed the defection of a Syrian air force MiG pilot who touched down in Jordan and won asylum, saying it was a "significant" moment and a sign of growing disquiet in the armed forces at Assad's scorched earth rule.

State Department spokeswoman Victoria Nuland said there was "no truth" to a report that Assad could be offered safe passage to Geneva for a conference on Syria that could take place this month.

"Issues of accountability in cases like this when you have grave human rights abuses against your own people are fundamentally a decision for the people of the country to make... we would not presume to speak for them."

White House spokesman Jay Carney said there was no sign that Assad was ready to exit and accept the power transition the United States demands.

Earlier, the London Times reported that Washington would launch a new push for Syrian peace following signs from Russian President Vladimir Putin that he believed Assad's time in power was receding.

The paper said the plan could see Assad get assurances he could travel unimpeded to Geneva to meet opposition parties.

The Guardian also said the United States and Britain were willing to offer safe passage and even clemency for Assad if he joined talks with rebels.

The leaks appeared to be a unilateral attempt by British officials to up pressure on Assad and to perhaps signal to the Syrian leader that it was not too late for him to step down.

They also held out the prospect that he could escape prosecution for crimes against humanity, as an incentive for him to leave power.

But the push did not seem to be coordinated with US officials, and any such initiative would raise the question over whether Assad's crackdown should disqualify him from any immunity.

The notion that Assad would leave Syria at a time of such turmoil also seemed far-fetched.

Human rights groups and others have called for the Syrian leader to be hauled before the International Criminal Court to account for the more than 15,000 deaths, mostly of civilians in his crackdown launched last year.

President Barack Obama said Tuesday that he hoped world powers could work with UN envoy Kofi Annan on a transition plan for Syria in the coming weeks,

But he also said that Russia and China, which have blocked tougher UN action against Assad, are not yet on board with the idea that Assad should go.

Putin said Tuesday it was not up to outsiders to decide who should lead other countries.

Russia is however making the point that it is not personally wedded to Assad, but is concerned about the possibility of an extremist takeover in post-transition Syria, sources said.

Russia is also guarding its strategic relationship with Damascus, and like China, is wary of any notion of regime change imposed by outside powers.

The focus of US efforts will shift to Secretary of State Hillary Clinton, due to meet Russian Foreign Minister Sergei Lavrov in Saint Petersburg next week.

Lavrov said Thursday that Assad would not accept any exile plan.

But Washington has been advancing a solution to the crisis modeled on a political transition in Yemen, under which President Ali Abdullah Saleh stepped down after an uprising but remained in the country and was granted immunity.

The defection of a Syrian MiG pilot gave US officials a rare moment of hope.

"It is obviously a significant moment when a guy takes a $25 million plane and flies it to another country," Nuland said. "This is how these things start."

National Security Council spokesman Tommy Vietor said the defection "will not be the last."

The New York Times meanwhile cited unnamed US officials and Arab sources as saying Central Intelligence Agency agents in Turkey were overseeing foreign weapons shipments and gathering information on Syria's fragmented opposition.

Officially, the US position is against further "militarizing" the conflict, amid concern in Washington about some extremist elements in the opposition and the destiny of weapons.

The report appeared as Obama faces significant domestic political pressure over his reluctance to arm the opposition.

In the past, the Obama administration had called for a complete arms embargo on Syria, but their position appeared to be shifting after failing to reach a consensus at the United Nations.

"We are keeping it on the table for the future but we need to make sure that if we go in that direction, it is a direction that will be supported by all," Nuland said.