Poland and the United States have signed an agreement to carry out a revised missile defence shield despite Russian objections.
US Secretary of State Hillary Clinton witnessed the signing of the pact, which allows the two nations to implement plans to station US missile interceptors on Polish soil to defend against potential threats from Iran and elsewhere.
Clinton then attended a conference on promoting democracy and civil liberties - an issue she is focusing on during a four-day trip to the former Soviet bloc nations of Ukraine, Poland, Azerbaijan, Armenia and Georgia.
US President Barack Obama decided last year to scrap a Bush-era plan to deploy a longer-range missile-defence system in Poland and the Czech Republic, opting instead to use smaller sea and land-based missile interceptors.
"The United States is deeply committed to Poland's security and sovereignty," Clinton said at a joint press conference with Polish Foreign Minister Radoslaw Sikorski.
"By signing an amendment to the ballistic missile defence agreement, we are reinforcing this commitment."
While there were reports last year that Poland was unhappy with Obama abandoning the Bush plan, Sikorski said his nation actually preferred the new approach.
"When President Obama announced the new configuration of the system, we did say that we liked the new configuration better but I think you didn't believe us," he said. "I hope now that we have signed the annex, I hope you do believe us."