The move has long angered Moscow, which perceives the development of the missile shield in its backyard as a hawkish move on the part of the US. Meanwhile, American officials have long maintained that the missile system is designed to defend the region against an Iranian missile threat.
But Mr. Obama has said that differences between Moscow and Washington over the European missile-defense system may be worked out through compromise.
That could mean that the architecture or location of the missile-defense system could look different if it is ever completed.
On Tuesday, Lt. Gen. Patrick O'Reilly, director of the Pentagon's Missile Defense Agency, said that alternatives to the current plan are under consideration as part of an overall review of ballistic missile defense.
General O'Reilly says the US would work "very closely" with the Polish and Czech governments to make any potential changes to the current plan. "There would not be any surprise," he told reporters.
For their part, the legislatures of Poland and the Czech Republic have yet to ratify the plan.
Obama has said the US review should be complete by the end of summer. It will look at "different courses of action," O'Reilly added Tuesday.
American officials could alter the current plan that relies on the ground-based interceptors in Poland, potentially exchanging them for sea-based interceptors or other kinds of missile-defense technology, defense officials said.
But critics of the plan say the issue goes beyond technology to the more fundamental issue of arms control.
"Missile defense has always been an issue for a small cadre of ultraconservative, ultraparanoid people who see the threat of ballistic missiles from places like Iran as a real threat to US security interests," says Chris Hellman, an analyst at the Center for Arms Control and Non-Proliferation in Washington.
Recent saber rattling by North Korea has brought missile defense to the fore, forcing the Obama administration to grapple with the European missile-defense plan.
So far, Obama has resisted the effort to link negotiations about the missile-defense system to arms-control talks. He has said he believes the US and Russia can find common ground in defending against missile strikes from a "third source" – namely Iran.
"I think we can arrive at those kinds of understandings, but it's going to take some hard work because it requires breaking down some long-standing suspicions," Obama said at a news conference with President Medvedev July 6 in Moscow.
Obama and Russian Mr. Medvedev issued a joint statement July 6 indicating the two countries were prepared to work with one another on missile defense.
Obama has said he wishes to "reset" US-Russian relations that have, he has said, "suffered from a sense of drift."
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