The United States has made a formal promise to Russia not to execute or torture Edward Snowden if he is sent home to face charges of illegally disclosing government secrets, and the Kremlin said Russian and US security agencies are in talks over his fate.

The 30-year-old former US spy agency contractor has been stuck in the transit area of a Moscow airport for more than a month despite Washington's calls to hand him over.

Russia has refused to extradite Snowden, who leaked details of a secret US surveillance programme including phone and Internet data, and is now considering his request for a temporary asylum.

In a letter dated July 23 and released on Friday, US Attorney General Eric Holder wrote that he sought to dispel claims about what would happen to Snowden if he is sent home.

"Mr Snowden has filed papers seeking temporary asylum in Russia on the grounds that if he were returned to the United States, he would be tortured and would face the death penalty. These claims are entirely without merit," Holder wrote.

Kremlin spokesman Dmitry Peskov said Russia's FSB and its US counterpart, the FBI, were in talks over Snowden, whose stay at the Moscow Sheremetyevo airport has further strained Moscow-Washington ties.

Russian President Vladimir Putin had expressed "strong determination", he said, not to let relations suffer over the dispute "no matter how the situation develops". Putin himself is not personally dealing with the problem, the spokesman said.

But he reiterated Moscow's stance that Russia "did not hand over, does not hand over and will not hand over anybody".

Putin, a former KGB spy, has said Snowden could only be granted sanctuary in Russia if he stopped actions that could harm the United States.

A US law enforcement official following the case confirmed the FBI has been in discussions with the FSB about Snowden for some time now but added he was not aware of any recent breakthroughs or imminent developments stemming from that.

A Russian security expert said the talks may be about how to secure a promise from Snowden to stop leaking if he were granted sanctuary in Russia.

"The United States maybe understands that they are not going to get Snowden, so my theory is that they are trying to save face and stop Snowden from publishing new exposes," Andrei Soldatov said, adding that he was skeptical this could be done.

ASYLUM OFFERS

Snowden's supporters have worried he could face the same fate as private first class Bradley Manning, the US soldier on trial for providing documents to WikiLeaks, the anti-secrecy group.. On his arrest, Manning was placed in solitary confinement for up to 23 hours a day with guards checking on him every few minutes.

"Torture is unlawful in the United States," Holder wrote, without explicit reference to Manning. "If he returns to the United States, Mr Snowden would promptly be brought before a civilian court."

Snowden has been offered asylum by three Latin American countries but none of them is reachable directly on commercial flights from Moscow, where he flew in from Hong Kong on June 23. He has also had his passport revoked by the United States.

His hopes for leaving the Sheremetyevo airport transit zone, which Russia insists is formally not its territory, were dashed at the last minute on Wednesday, prompting a wave of speculation about possible political intervention or a hitch.

A Russian lawyer assisting Snowden in his asylum request, Anatoly Kucherena, who also sits on an advisory group to the Russian authorities, said his client feared he could face torture or the death penalty if returned to the United States.

Russia's federal migration service has up to three months to consider Snowden's temporary asylum request filed on July 16. An official was quoted on Friday as saying that could be extended to six months.

Snowden is a convenient propaganda tool for the Kremlin, which often accuses the United States of preaching abroad what it does not practice at home on human rights.

But Moscow has also held Snowden at arm's length as Putin wants US President Barack Obama to come to a bilateral summit in Moscow before a G20 meeting in St Petersburg in September.

The White House has left it vague on whether Obama will come to the face-to-face talks.

A US Senate panel voted unanimously on Thursday to seek trade or other sanctions against Russia or any other country that offers asylum to Snowden.

The US House of Representatives on Wednesday narrowly rejected a plan to limit the National Security Agency's ability to collect electronic information, including phone call records.

Snowden's father on Friday blasted US lawmakers for not reining in the electronic spy program made public by his son, accusing them of being "complicit or negligent."

Bruce Fein, an attornery for Snowden's father Lonnie, said he had not yet received a response from Holder to a letter he sent to the attorney general suggesting developing "parameters for a fair trial" for Edward Snowden. Fein said Lonnie Snowden had not had any direct communication with his son since April. - Reuters