Romania on Monday began constructing a facility that will host missile interceptors as part of a planned NATO missile shield, a project that has irked Russia.
"This is an historic occasion", NATO deputy secretary-general Alexander Vershbow said during the groundbreaking ceremony at the Deveselu military base in southern Romania.
The event was attended by US Under Secretary of Defence for Policy James N. Miller, as well as Romanian President Traian Basescu and James Syring, director of the US Missile Defence Agency.
"The facility here in Deveselu will be a crucial component in building up NATO's overall ballistic missile defence system," Vershbow said. "By the end of 2015 this base will be operational and integrated into the overall NATO system."
Launched in 2010, NATO's missile shield project, based essentially on US technology, will see the progressive deployment of missile interceptors and powerful radars in Eastern Europe and in Turkey.
The project -- said to be aimed at countering a potential Iran threat -- has drawn opposition from Russia, which fears it could compromise its security.
Washington will invest $134 million (97 million euros) in the Deveselu base, built some 60 years ago with the Soviet Union's help.
The airbase, which will remain under Romanian command, will host an average of 200 US troops, up to a maximum of 500.
"What we are initiating here today is a powerful and tangible manifestation of our solemn commitment to mutual security in Europe", said Miller.
Scheduled to become fully operational in 2018, the missile defence system will be deployed over several years.
Poland and Romania have agreed to host 24 US land-based SM-3 missiles interceptors each in the coming years. US ships equipped with missile interceptors are to be based in a Spanish port.
President Basescu on Monday said the shield was aimed at "protecting European and American citizens".