The United States has made a formal promise to Russia not to torture or execute whistle-blower Edward Snowden if he is sent home to face charges of illegally disclosing government secrets.
The news emerged as the Kremlin said Russian and US security agencies were in talks over the 30-year-old former US spy agency contractor's fate.
Snowden has been stuck in the transit area of Moscow's Sheremetyevo 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 operation including phone and internet data, and is considering his request for temporary asylum.
In a letter sent on Tuesday and released yesterday, 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 Sheremetyevo airport has further strained Moscow-Washington ties. Russian President Vladimir Putin had expressed "strong determination", he said, not to let relations suffer "no matter how the situation develops".
Putin, a former KGB spy, has said Snowden could be granted sanctuary in Russia only 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 breakthroughs or imminent developments.
Russian security expert Andrei Soldatov said the talks might be about how to secure a promise from Snowden to stop leaking secrets if he were granted sanctuary in Russia.
"The US maybe understand 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," Soldatov said, adding that he was sceptical this could be done. 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.
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 US.
His hopes of leaving 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 yesterday as saying that could be extended to six months.