Technical problems will delay the completion of the Polish section of a U.S. defence shield by two years to 2020, Poland's defence ministry said on Thursday.
The anti-missile site close to Poland's northern Baltic Sea coast is part of a NATO defensive umbrella that, when complete, will stretch from Greenland to the Azores.
Poland's defence ministry said U.S. authorities had told it there had been construction delays at the Redzikowo site that were the responsibility of the contractor.
Officials from the US-led NATO alliance, who took control of the umbrella in 2016, have said the whole system is supposed to defend against attacks by states such as Iran and groups such as al Qaeda.
But the scheme has angered Russia, incensed by the show of force by its old Cold War rival in formerly communist Eastern Europe.
Poland's former defence minister, Tomasz Siemoniak, from the opposition party Civic Platform (PO) party said the delay was meant as a political rebuke to the current government.
"Unofficially, it is an obvious price for the ... lack of trust in PiS (the ruling conservative Law & Justice party)," Siemoniak said on Twitter.
Bartosz Cichocki, deputy foreign minister, told the PAP state news agency that there is no crisis in Polish-American relations.
As part of the shield, the United States switched on an $800 million site in Romania in May 2016 and broke ground at the site in the Poland later that year.
The full shield also includes ships and radars across Europe, with command and control run from a U.S. air base in Germany.
The U.S. Missile Defence Agency was not immediately available to comment.