Russian President Vladimir Putin has warned that US plans to build a missile defence system in eastern Europe would raise the risk of "mutual destruction".
Mr Putin has made a series of comments against missile defence
Poland and the Czech Republic are keen to allow the US to site missile bases and radars on their territory.
Mr Putin spoke a day after threatening to halt involvement with a treaty limiting conventional arms in Europe.
"The threat of causing mutual damage and even destruction increases many times," he told Russian media.
"This is not just a defence system, this is part of the US nuclear weapons system," the Itar-Tass news agency quoted him as saying after meeting Czech President Vaclav Klaus.
Speaking at a press conference with Polish President Lech Kaczynski in Warsaw, British Prime Minister Tony Blair insisted the missile defence plan was not aimed at Russia.
"I am absolutely sure myself that it is not in any shape or form aimed at Russia or as a consequence of issues to do with America or Europe's relations with Russia," Mr Blair said.
"I think it's more to do with the concern over... those states that are trying to acquire nuclear weapons capability for the first time."
Mr Putin has taken a tough line in recent months over the US plans for missile defence.
His suggestion on Thursday that Russia could suspend membership of the 1990 Conventional Forces in Europe (CFE) treaty was met with "grave concern" by Nato.
Nato Secretary General Jaap de Hoop Scheffer said the agreement was one of the cornerstones of European security.
Mr Putin has accused the US of overstepping its "natural borders" and of his concern at the apparent increase in military bases and systems close to Russia's borders.
As part of the its new missile defence programme, the US now wants to station 10 interceptor missiles in Poland, with radar operations in the Czech Republic.
Mr Putin's use of the term "mutual destruction" harks back to the rhetoric of the Cold War, when strategists in Russia and the US relied at least partly on the theory of Mutually Assured Destruction (MAD) to prevent nuclear war.
The theory underpinned the Anti-Ballistic Missile (ABM) treaty of 1972, which limited the development of anti-missile systems.
But the US withdrew from the ABM treaty in 2002, calling it a "relic" from a previous age.