MOSCOW -- The United States has effectively canceled the final phase of a Europe-based missile defense system that was fiercely opposed by Russia and cited repeatedly by the Kremlin as a major obstacle to cooperation on nuclear arms reductions and other issues.
Russian officials here have so far declined to comment on the announcement, which was made in Washington on Friday by Defense Secretary Chuck Hagel as part of a plan to deploy additional ballistic missile interceptors to counter North Korea. The cancellation of some European-based defenses will allow resources to be shifted to protect against North Korea.
Aides to President Vladimir V. Putin of Russia said there would be no reaction until early next week, when they expect to be briefed by American officials.
But Russian news accounts quickly raised the possibility that the decision could portend a breakthrough in what for years has been a largely intractable dispute between Russia and the United States. A headline by the Itar-Tass news agency declared, "U.S. abandons fourth phase of European missile defense system that causes the greatest objections from Russia."
Russian leaders on several occasions used meetings with President Obama to press their complaints about the missile defense program. At one such meeting, in South Korea last March, Mr. Obama was heard on a live microphone telling the outgoing Russian president Dmitri A. Medvedev in a private aside that he would have "more flexibility" to negotiate on missile defense after the November presidential election in November.
Pentagon officials said that Russia's longstanding objections played no role in the decision to reconfigure the missile interceptor program, which they said was based on the increased threat from North Korea and on technological difficulties and budget considerations related to the Europe-based program.
"The missile defense decisions Secretary Hagel announced were in no way about Russia," George Little, a Pentagon spokesman, said Saturday.
Still, other Obama administration officials acknowledged potential benefits if the decision was well-received in Moscow, as well as the possibility of continued objections given that the United States is not backing away from its core plan for a land-based missile shield program in Central Europe.
"There's still an absolutely firm commitment to European missile defense, which is not about Russia; it's about Iran these days," said a senior administration official who spoke on the condition of anonymity. "If there are side benefits that accrue with Russia, so be it. But that wasn't a primary driver."
Regardless, some experts said it could help relations by eliminating what the Russians had cited as one of their main objections -- the interceptors in the final phase of the missile shield that might have the ability to target long-range intercontinental ballistic missiles, which are part of Russian's nuclear arsenal.
The Obama administration has sought cooperation from Russia on numerous issues, with varying degrees of success. Russia generally has supported the NATO-led military effort in Afghanistan and has helped to restrict Iran's nuclear program by supporting economic sanctions. But the two countries have been deeply at odds over the war in Syria, and over human rights issues in Russia. Most recently, Mr. Obama has said he would like further reductions in the two countries' nuclear arsenals, something Russia has said it would not consider without settling the dispute over missile defense.
American experts insisted that the Russians' concern over the antimissile program was exaggerated and that the system would not have jeopardized their strategic missiles had the final phase been developed. That Russian concern has now been addressed.
"There is no threat to Russian missiles now," said Steven Pifer, an arms control expert who has managed Russia policy from top positions at the State Department and the National Security Council. "If you listen to what the Russians have been saying for the last two years, this has been the biggest obstacle to things like cooperation with NATO."
"Potentially this is very big," said Mr. Pifer, now of the Brookings Institution. "And it's going to be very interesting seeing how the Russians react once they digest it."
In Washington, many officials have said they believe Russia's real objections are not only about the particular capabilities of the missile shield but also about a more general political and strategic opposition to an expanding American military presence in Eastern Europe. Canceling only the final stage of the program does not address that concern, so it is possible that Russia's position will remain unchanged.
Sean Kay, a professor at Ohio Wesleyan University and expert in international security issue and Russian relations, said that the so-called fourth stage of the Europe-based missile defense program "was largely conceptual" and might never have been completed.
Eliminating that portion of the program made sense, Mr. Kay said. "In effect, by sticking with a plan that was neither likely to work in the last stage but was creating significant and needless diplomatic hurdles at the same time, we gained nothing," he said. At least some of the canceled interceptors were to have been based in Poland, which will still host less-advanced interceptors.
In the past, efforts to restructure the antimissile program provoked sharp criticism in Poland, but this time reaction from Warsaw has been more muted. Analysts have said Poland's main goal in hosting the interceptors has been having an American military presence there as a deterrent to Russia.
David M. Herszenhorn reported from Moscow, and Michael R. Gordon from Washington. Thom Shanker contributed reporting from Washington.
This article originally appeared in The New York Times.