WASHINGTON — The Obama administration is shelving a European missile defense plan that has been a major irritant in relations with Russia, a U.S. ally said this morning. The Pentagon confirmed a "major adjustment" is planned.

Jan Fischer, the prime minister of the Czech Republic, one of two countries where the system was to be built, told reporters that Obama phoned him overnight to say "his government is pulling out of plans to build a missile defense radar on Czech territory."

Without giving specifics, Pentagon spokesman Geoff Morrell said, "We have made a major adjustment and enhancement to our European missile defense system that will better protect our forces deployed in Europe and our allies there from Iranian short- and medium-range missiles."

He said the change comes in part because the U.S. has concluded that Iran is less focused on developing the kind of long-range missiles for which the system was originally developed.

"While the Iranian threat has developed, so too has our technology," Morrell said. Details were expected to be announced later today.

Obama's top military adviser, chairman of the Joint Chiefs of Staff Adm. Mike Mullen, told The Associated Press on Wednesday the administration was "very close" to the end of a seven-month review of a missile defense shield proposal, an idea that was promoted by the George W. Bush administration. Mullen would not divulge its results.