WASHINGTON -- President Barack Obama announced Thursday that he's canceling the Bush administration's controversial missile defense shield in Eastern Europe, citing new U.S. intelligence estimates that show the threat from long-range Iranian ballistic missiles is growing more slowly than previously thought.

Obama, citing a Pentagon review of the program, said he'd proceed instead with a cheaper and less diplomatically sensitive anti-missile program that would, at least in its early phases, be based aboard U.S. Navy ships at sea.

The announcement doesn't affect other aspects of the U.S. missile defense program, including ground-based interceptor missiles deployed at Alaska's Fort Greely and in California.

Russia had angrily opposed the Bush administration's plans to deploy missile interceptors in Poland and a missile-tracking radar in the Czech Republic.

The new White House plan appeared aimed in part at gaining Moscow's cooperation in curbing Iran's suspected nuclear-weapons program and boosting U.S.-Russian talks on a new strategic arms treaty. Senior U.S. officials, however, denied seeking any quid pro quo.

Obama's move caused consternation in Poland and the Czech Republic, two close post-Cold War U.S. allies and former Soviet satellites whose governments had agreed to host the missile defense sites despite popular opposition.

Obama said he'd spoken to the prime ministers of both countries and reaffirmed the U.S. commitment to their security.

In what appeared to be a hastily scheduled White House appearance after overnight leaks of his decision, Obama said the new systems he was proposing "will provide stronger, smarter and swifter defense of American forces and America's allies."

He said his decision was based in part on the updated intelligence assessment of Iran's missile programs, which emphasized its progress on short- and medium-range weapons, as opposed to long-range ones.

Republicans, who've advocated missile defenses since President Ronald Reagan's 1980s Strategic Defense Initiative, dubbed "Star Wars," swiftly denounced Obama's decision as a retreat and an invitation for Russia to coerce its former satellites.

The decision "does little more than empower Russia and Iran at the expense of our allies in Europe," said House Republican leader John Boehner of Ohio. "It shows a willful determination to continue ignoring the threat posed by some of the most dangerous regimes in the world, while taking one of the most important defenses against Iran off the table."

His Senate counterpart, Sen. Mitch McConnell of Kentucky, complained that Obama "has secured no apparent commitment from the Russians to work with us to reduce either the missile or nuclear threat from Iran. More troubling will be if the administration has made these concessions to Russia in pursuit of expediting ill-considered arms control deals."

Administration spokesmen said that Russian views didn't drive the new missile defense strategy, which they said was based on estimates of the Iranian threat, cost and technology.

"This is not about Russia," White House press secretary Robert Gibbs said. Obama wanted to deploy a system that was "both technologically and cost effective. We did not want to deploy something that didn't work," Gibbs said.

Defense Secretary Robert Gates said in a news briefing that the new approach would allow the United States to deploy missile defenses sooner and would make it harder for Russia to continue to argue that the system was aimed against it.

The Bush approach called for land-based missile interceptors to take out ballistic missiles launched from Iran, with the help of tracking radars. The system Obama is proposing would be based, at least in the first phase, aboard Navy ships equipped with an anti-missile weapon known as the SM-3. SM stands for Standard Missile.

Sen. Lisa Murkowski, R-Alaska, echoed the Republican leadership, calling Obama's decision "regrettable." "I just don't understand the message they are trying to send," she said in a written statement. "To rogue states that are building their offensive nuclear capabilities the message from our Pentagon signals vacillation. To our allies in the former Soviet satellites, such as Poland and the Czech Republic, the message is 'don't count on the United States for consistency in our strategy for your defense.' When it comes to missile defense our resolve seems to turn on a dime.

Sen. Mark Begich, D-Alaska, said the move makes the Alaska-based missiles more important than ever. He said he was looking forward to hearing how the administration "proposes to maintain" the Alaska system. "Assuming the President's decision to cancel missile system deployment in Europe is final, Alaska's GMD system must be fully deployed and properly maintained to protect America from attacks by rogue states," he said in a statement.