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.”