US Secretary of State Hillary Clinton has overseen the signing in Poland of a key missile base agreement.
The deal sets up a permanent United States missile base as part of a revamped version of a missile defence shield.
Russia opposes a Polish base hosting US missiles, but Mrs Clinton says the new deal does not threaten Russia.
Mrs Clinton arrived from Ukraine, which was the first stop on a tour of eastern European states. She will later visit Azerbaijan, Armenia and Georgia.
Mrs Clinton witnessed the signing of the amendment to the agreement with her Polish foreign minister Radoslaw Sikorski in Krakow.
Mrs Clinton said the agreement would "protect Poland and our allies from evolving threats, such as those from Iran".
Poland previously agreed with the Bush administration to host a permanent US military base and missiles at a disused airstrip near the Baltic Sea.