US threats that China and Russia face "consequences" if leaker Edward Snowden evades capture may prove just hot air, experts say, with Washington powerless in a game of cat-and-mouse.

Left red-faced after Snowden brazenly waltzed out of Hong Kong bound for Moscow at the weekend even after his passport was apparently canceled, US officials have angrily called on Russia to hand him over for trial.

President Barack Obama said Washington was using every legal channel to apprehend the former technician and the self-confessed source of explosive leaks detailing the extent of covert US phone and Internet surveillance.

Just two weeks after Obama cozied up with Chinese President Xi Jinping in a summit in California, White House spokesman Jay Carney lashed out at Beijing for letting Snowden go despite US extradition requests.

"We are just not buying that this was a technical decision by a Hong Kong immigration official," Carney said.

"This was a deliberate choice by the government to release a fugitive despite a valid arrest warrant, and that decision unquestionably has a negative impact on the US-China relationship."

On Monday, US Secretary of State John Kerry, who has branded Snowden a traitor, warned of consequences to US-Sino and US-Russian ties saying it was "deeply troubling" if requests for his extradition were ignored.

But in reality, experts say there is little that Washington can do to force Moscow to return Snowden or even to persuade Ecuador and Venezuela, both possible places of refuge, to deny him shelter.

"I am skeptical," acknowledged Michael O'Hanlon, director of research for foreign policy at the Brookings Institution, saying that "the United States will not have an interest in any kind of major retaliation."

While he told AFP he did not approve of Snowden's actions, he added: "I am not convinced this rises to a major security revelation or crisis and I am not convinced that Russia and China have done anything out of the ordinary with normal protocol."

With major international problems from Syria to Iran on the global agenda, the US might just be unwilling to see the Snowden affair explode into a full-blown diplomatic crisis.

US officials said Monday afternoon Snowden was believed to be still holed up in Moscow, as top officials including US ambassador to Moscow Michael McFaul and senior FBI leaders worked frantically to win his arrest.

"The fact that he is now in Moscow and did not get on the airplane for Cuba tells me that the old KGB officer -- now president of Russia, Putin -- is directing the show," said US Senator Bill Nelson.

Nelson added that he imagined that Vladimir Putin was probably "giving orders to milk him for every piece of information that he has."

"If he really doesn't have anything, then I think the president of Russia is going to decide whether or not he wants to have a good relationship with the United States and might allow him to be extradited to the US," he said.

Their pride somewhat bruised, US officials have noted that Snowden appears to be seeking refuge in countries not known for their freedom of speech.

"There is a certain irony here, of course, that somebody who says that he's about freedom of the Internet and freedom of information, of course, would seek out some of these countries in particular," said State Department spokesman Patrick Ventrell.

"You don't see him standing up for the free flow of information in some of these countries that don't always have that."

And certainly nations like Ecuador and Venezuela, which have prickly ties at best with the US, could seize on the moment to deal a blow to Washington in a game of diplomatic one-upmanship.

"In the case of Ecuador, there is once again, like Venezuela, a willingness and even an eagerness to kick sand in Uncle Sam's face," said Cynthia Arnson, director for the Latin American program at the Woodrow Wilson International Center for Scholars.

One direct consequence for Quito could be the end of some trade preferences granted by Congress, she said.

"I can imagine the US government taking steps to expel the Ecuadoran ambassador," she added.

"In the case of Venezuela it certainly runs contrary to what seems to be the interests of (President Nicolas) Maduro's government in improving the relationship with the United States."