US President George W Bush has said Russia has nothing to fear from a missile defence system to be built by the US, partly in eastern Europe.
Mr Bush said the Cold War was over and Russia was not an enemy of the US.
The president was speaking during a visit to the Czech Republic, en route to the G8 summit in Germany.
Russian President Vladimir Putin has threatened to aim weapons at Europe if the shield is set up, in comments seen as reminiscent of Cold War rhetoric.
Washington has dismissed talk of a new Cold War but says the language coming out of the Kremlin has worrying echoes of that era.
In a news conference with the Czech president and prime minister, Mr Bush said the new system was a "purely defensive measure, aimed not at Russia but at true threats".
Washington wants to deploy interceptor rockets in Poland and a radar base in the Czech Republic to counter what it describes as a potential threat from "rogue states" such as Iran and North Korea.
He said he would invite Russia to cooperate with the US over the system. Mr Bush is expected to meet Mr Putin at the G8 summit in Germany on Wednesday.
"Russia is not our enemy," Mr Bush said.
"I look forward to having conversations with President Putin. My message will be - you should not fear the missile defence system.
"Why don't you cooperate on the missile defence system? Why don't you participate with the United States? Send your generals over to see how such a system would work, send your scientists."
But Russian Foreign Minister Sergey Lavrov dismissed the perceived threat from Iran and North Korea.
"What we all need is to join our efforts to fight real, not hypothetical, threats. And for this work, Russia is ready," he said, quoted by Reuters news agency.
On Sunday President Putin said Iran was not a threat to the US, hinting that Russia was the target of the new US system.
"If the American nuclear potential grows in European territory, we will have to have new targets in Europe," Mr Putin said.
Speaking on the way to Europe, Mr Bush's national security adviser, Stephen Hadley, said Mr Putin's remarks were "not helpful".
Nato spokesman James Appathurai described Mr Putin's words as "unwelcome", while new French President Nicolas Sarkozy said he would have "frank" talks with the Russian leader.
Polish Prime Minister Jaroslaw Kaczynski, meanwhile, said Mr Putin's comments were reminiscent of the Cold War rhetoric of Nikita Khrushchev, Soviet leader in the 1950s and 60s.
"This is language which has never been used by [Boris] Yeltsin, nor [Mikhail] Gorbachev and not even [Leonid] Brezhnev... This is the language of Khrushchev," he said.
Mr Putin's spokesman has since attempted to soothe the row, describing the comment as a "hypothetical" response to a "hypothetical" question.
Mr Putin issued his warning in an interview with foreign reporters ahead of the G8 meeting in Heiligendamm, Germany, on Wednesday.
He hoped US officials would change their minds about the missile plan, and said that if an arms race resulted it would not be Russia's fault.
Last week, Moscow announced it had tested a ballistic missile to maintain "strategic balance" in the world.