MOSCOW/WASHINGTON - 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 Tuesday 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.