WASHINGTON (AP) — President Barack Obama on Thursday shelved a Bush-era plan for an Eastern European missile defense shield that has been a major irritant in relations with Russia. He said a redesigned defensive system would be cheaper and more effective against any threat from Iranian missiles.

Anticipating criticism from the right that he was weakening America's security, Obama said repeatedly that this decision would provide more protection, not less.

It is more comprehensive than the previous program, it deploys capabilities that are proven and cost effective, and it sustains and builds upon our commitment to protect the U.S. homeland," Obama said in a statement released by the White House.

With the announcement, Washington scrapped what had become a politically troublesome plan, and one the Defense Department says was ill-suited to what the United States considers the true threat from Iran. In its place would be a system the Pentagon contends would accomplish the original goal and more.

Defense Secretary Robert Gates said Iran's changing capabilities drove the decision, but he acknowledged that the replacement system probably will allay some of Russia's concerns.

The change comes days before Obama is to see Russian President Dmitry Medvedev for meetings at the United Nations and the Group of 20 nations economic summit.

The plan for a European shield was a darling of the Bush administration, which reached deals to install 10 interceptors in Poland and a radar system in the Czech Republic, both eastern European nations at Russia's doorstep and once under Soviet sway. Moscow argued vehemently that the system would undermine the nuclear deterrent of its vast arsenal.

Its concerns about our previous missile defense programs were entirely unfounded," Obama said, speaking of Russia.

Obama phoned Czech Prime Minister Jan Fischer Wednesday night and Polish Prime Minister Donald Tusk Thursday morning to alert them of his decision. It was not clear whether Washington also informed Russia.

It was unclear whether any part of the future system would be hosted by those nations, which agreed to handle the Bush-planned shield at considerable cost in public opinion and their relations with Russia. Gates said they might, and said he hopes Poland still will approve a broad military cooperation agreement with the United States.

Criticism came immediately from Republicans.

Rep. Eric Cantor of Virginia, the second-ranking Republican in the House of Representatives, said he would "work to overturn this wrong-headed policy.

Scrapping our missile defense effort in Europe has severe consequences for our diplomatic relations and weakens our national security," Cantor said in a statement. "Our allies, especially Poland and the Czech Republic, deserve better, and our people deserve smarter and safer.

House Speaker Nancy Pelosi, a Democrat and the chamber's top official, called the policy change "brilliant.

That they have re-evaluated the threat; that they have taken into consideration what the technology — advanced technologies are now; what this means to our relationship in NATO; and what this means in our bilateral relationship with Russia is very, very important.

The new plan would rely on a network of sea- land- and air-based sensors and interceptor missiles as a bulwark against Iranian short- and medium-range missiles.

The Bush missile shield plan, which never moved beyond the blueprint stage, would have been a deterrent for Iranian long-range missiles, but Russians worried that the system would be aimed at them.

Gates said the initial stage of Obama's alternate plan would deploy Aegis ships armed with interceptors, giving the military the ability to move the system around.

Another key to the near-term network would be new, more mobile radar used to detect and track short- and medium-range missiles that had been launched from Iran.

Gates said a second phase of the plan would add a modified version of a land-based missile that would not be ready until 2015.

Gates said the decision to abandon the Bush administration's plans came about because of a change in the U.S. perception of the threat posed by Iran. U.S. intelligence decided short- and medium-range missiles from Iran now pose a greater near-term threat than the intercontinental ballistic missiles the Bush plan dealt with, he said.

Still, the decision can be read at least in part as an effort to placate Russia at a time when the United States badly wants Russian cooperation in an international pressure campaign, and perhaps new tough sanctions, against Iran's suspected nuclear program.

Russia remains opposed to any new sanctions against Iran. Before Obama spoke on Thursday, Russian Foreign Minister Sergey Lavrov pointedly said tougher sanctions would ruin hopes for resolving concerns over the Iranian nuclear program through talks.

NATO Secretary-General Anders Fogh Rasmussen called the U.S. decision "a positive step.

And Konstantin Kosachev, head of the foreign affairs committee in the lower house of the Russian parliament, said, "It reflects understanding that any security measure can't be built entirely on the basis of one nation."