"There has been some escalation in the rhetoric. We think that is not helpful," said U.S. National Security Adviser Stephen Hadley while accompanying U.S. President George W. Bush on his trip to Europe.

"We would like to have a constructive dialogue with Russia on this issue. We have had it in the past," Hadley said.

Putin warned on Monday that Russia would take retaliatory measures against the deployment of a U.S. missile defense system in Eastern Europe.

"In case of the deployment of anti-missile defense systems in Europe we warn that there will be retaliatory steps. We have to ensure our security. And this process was not initiated by us," Putin said in an interview.

The United States plans to deploy missile interceptors in Poland and radar units in the Czech Republic as part of a project to extend the missile defense system in Europe. Russia has voiced strong opposition over the deployment.