The European sites, designed to counter an emerging ballistic missile threat from Iran, would have consisted of 10 interceptor missiles based in Poland, and a powerful targeting radar known as X-band radar in the Czech Republic.

Those sites would have been part of a more extensive US system that until now has been centered in Alaska and California and designed to provide a limited defense against a North Korean long-range missile attack.

The system would have relied on a complex network of satellite sensors and ground- and sea-based radars to detect and track launches of long-range missiles.

A command center in Colorado Springs, Colorado, would have integrated missile launch and trajectory data to cue interceptor missiles and guide them into a collision in space with an incoming warhead.

The system currently consists of 21 interceptor missiles in Alaska and three interceptor missiles at Vandenberg Air Force Base in California.

A network of early warning satellites provides the first alert of a launch, which is then picked up by a variety of radars.

These include tracking radars aboard Aegis warships, a forward-deployed X-band radar in Japan, a huge X-band radar on a floating platform in the Pacific, and upgraded early warning radars in Alaska and California.

Upgraded early warning radars in Thule, Greenland, and Fylingdales, United Kingdom also were to be integrated into the system.

Pentagon officials had said that the X-band radar in the Czech Republic and interceptor missiles in Poland would extend the system's coverage to most of Europe for the first time.

They said the sites in the Czech Republic and Poland were selected because they are optimal for defending against a missile attack from Iran.

Russia vehemently opposes the system on grounds that it could be turned against its nuclear arsenal, but US officials deny that.

If construction had begun in 2009, the European site would have been ready for operations between 2011 and 2013.