US Defense Department experts say plans to build a missile system in Eastern Europe could be delayed as the interceptors have not been tested.
The concerns were raised in an October report by the Defense Department's internal testing oversight arm, the Operational Test and Evaluation Directorate, which rejected the claims that previous, successful tests completed on a US-based missile defence system were applicable to the European system.
Administration officials initially disregarded the findings and reassured lawmakers the system to intercept missiles would work, AP reports.
But with Congress now poised to require additional tests, the department has reversed this view and is planning three trial interceptor launches — a process that could take years.
"Operations in the European theatre present new challenges that must be fully investigated to ensure its overall effectiveness in carrying out the European missile defence mission," the report said.
The plans would place ten interceptors in Poland, which, in conjunction with radars in the Czech Republic, could identify and shoot down missiles fired by Iran at Europe or the US.
Russia strongly opposes basing the interceptors across its border and claims the system's real target would be Russian missiles.
The deployment of the missile launchers has not yet been agreed to by the Polish Government.
Last week, Lithuania indicated it may be open to the installation of such facilities if Poland will not agree.
Secretary of State Condoleezza Rice is expected to sign an agreement with the Czech Government to site an anti-missile radar in the country early next month.
Thereafter, the two accords will still have to be ratified by the Czech Parliament, AFP reports.
The US hopes to have the system installed by 2013.
By Elizabeth Clifford-Marsh