Former Cold War foes in Europe are holding talks in Vienna to review a key arms control agreement which has put Russia and Nato at loggerheads.
Russia's intervention in Chechnya is among the thorny issues
The Conventional Forces in Europe (CFE) Treaty, signed in 1990, limits the number of conventional weapons countries are allowed.
But the Russians say it is outdated and restricts their ability to move troops around their own territory.
President Vladimir Putin has announced a moratorium on honouring the treaty.
The Russian leader says the CFE treaty no longer reflects the post-Cold War world.
The head of the US delegation at the talks, Daniel Fried, said the treaty could be preserved successfully into the 21st Century and it would be unfortunate if the Russians carried out their threat to halt compliance.
The CFE treaty was adapted in 1999, eight years after the Warsaw Pact was dissolved.
Russia ratified the revised version, but Nato states have not done so, first demanding the withdrawal of Russian forces from Georgia and Moldova.
The BBC's diplomatic correspondent Jonathan Marcus says the CFE treaty harks back to another age and in many ways it is difficult to see quite why it is relevant today.
Russia was alarmed when the US withdrew from another Cold War agreement - the Anti-Ballistic Missile Treaty - in 2001.
Faced by a US administration that sees less and less value in such agreements, the Russians are signalling that what goes for Washington goes for them too, our correspondent says.