U.S. President Barack Obama is dropping plans for a missile defense system with interceptors in Poland and a radar facility in the Czech Republic. It is a dramatic reversal of a Bush administration policy that created deep tensions with Russia.

President Obama says he is scrapping the Bush administration's proposal, and replacing it with a plan that is more flexible and effective.

"The best way to responsibly advance our security and the security of our allies is to deploy a missile defense system that best responds to the threats that we face and that utilizes technology that is both proven and cost effective," President Obama said.

He says his decision was guided in large part by new intelligence that shows Iran is focusing on the development of medium and short-range missiles.  The original missile defense policy was based on the assumption that Tehran was seeking a long-range capability.

"This new ballistic missile defense program will best address the threat posed by Iran's ongoing ballistic missile defense program," the president said.

Mr. Obama says the change in policy was also prompted by advances in technology, including improvements in land and sea-based interceptors and the sensors that support them.

He says these improved systems will be gradually deployed, starting in about 2011.   Under his plan the large centralized radar facility proposed for the Czech Republic will not be needed, nor will the earlier generation of ground-based interceptors planned for Poland.

"To put it simply: our new missile defense architecture in Europe will provide stronger, smarter and swifter defenses of American forces and America's allies," he said.

President Obama made the announcement at the White House after first informing the leaders of Poland and the Czech Republic.  He said the decision to drop the old missile defense plan in no way lessens America's commitment to their security.

The policy shift is sure to be on the agenda next week when Mr. Obama is likely to meet, at either the United Nations or the Group of 20 economic summit, with Russian President Dmitiri Medvedev.

Plans for a missile defense system in parts of the former Soviet bloc have been an irritant between Washington and Moscow.  And in making the announcement, President Obama said once again the American drive for a missile shield has never had anything to do with Russia.

"Our clear and consistent focus has been the threat posed by Iran's ballistic missile program and that continues to be our focus and the basis of the program that we are announcing today," President Obama said.

Republican leaders in the U.S. Congress were quick to criticize President Obama's decision.  In a written statement, the top Republican in the House of Representatives, John Boehner of Ohio, accused the White House of empowering Russia at the expense of America's European allies.