America has 56,000 troops in Afghanistan
A senior Obama administration official has told the BBC that Russia has agreed to let US troops bound for the war in Afghanistan fly through its airspace.
The deal, which opens up an important new corridor for the US military, is to be officially announced when President Barack Obama visits Moscow next week.
Speaking separately, a Kremlin official confirmed a deal was on the table but suggested it referred to weapons only.
The reported agreement marks a major development in US-Russian relations.
Until now Russia has restricted use of its territory for the Afghan conflict, only allowing the US to transport non-lethal supplies to Afghanistan by train, the BBC's Jon Donnison reports from Washington.
The Obama official who spoke to the BBC said that, under the new agreement, US military planes carrying weapons as well as troops would be allowed to make thousands of flights a year through Russian airspace.
In recent years, Moscow and Washington have not seen eye to eye, with disagreements over Nato expansion into Eastern Europe, Russia's conflict with Georgia and America's plans for a missile defence shield.
This new co-operation over the Afghan war could pave the way for an improvement in diplomatic relations, our correspondent says.
'No troop request'
Mr Obama is due to visit Moscow between 6 and 8 July when he and his Russian counterpart, Dmitry Medvedev, are due to discuss reducing each other's nuclear stockpile, as well as Iran and North Korea.
Sergei Prikhodko, Mr Medvedev's top foreign policy adviser, said on Friday that the two presidents planned to sign a "joint agreement on military transit to Afghanistan".
Transit of weapons would be by both land and air but mostly by air, he said.
He added that it was unclear if US soldiers or other personnel would be permitted to travel through Russian territory or airspace.
"They haven't asked us for it," he told reporters at the Kremlin.
The Kremlin adviser added:
"Today we sense a desire of our American partners to combine wide co-operation... with a readiness to breathe new life into bilateral trade and economic cooperation."
America's normal supply route to Afghanistan via Pakistan has come under repeated militant attack, and the US and Nato are keen to find alternative supply routes through Russia and the Central Asian states.
The US has about 56,000 troops in Afghanistan while its Nato partners have some 32,000 deployed.
Russia's powerful Prime Minister, Vladimir Putin, has urged the US to move relations forward by shelving plans for a missile defence shield in Europe.
He also fended off a call by Mr Obama this week for Russia to end "old Cold War approaches" to relations. Replying to the US leader's suggestion that he, Mr Putin, had one foot in the past, he said:
"We [Russians] don't stand bowlegged. We stand solidly on our own two feet and always look to the future".