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The concerns were heightened by a section of the State Department internal report describing the Pentagon’s new Phased Adaptive Approach to missile defense in Europe. The report, which was produced after last month’s NATO summit in Lisbon, made no mention of the fourth and final phase of the European missile defense program.

The omission prompted new concerns that the Obama administration may have put the fourth phase on hold to avoid upsetting the Russians, congressional aides said.

The fourth phase calls for deploying 48 advanced silo-based versions of the SM-3 anti-missile interceptor by 2020 for use against an anticipated Iranian intercontinental ballistic missile. The 48 SM-3s would replace the 10 ground-based interceptors planned for a site in Poland that was canceled after Moscow voiced opposition to it.

Three Republican senators wrote to President Obama on Monday asking about recently announced plans by NATO to include Russia in a program of missile defense cooperation, specifically whether Moscow will gain access to “some our country’s most sensitive technology, collection assets and real-time intelligence.”

The lawmakers, led by Sen. Jon Kyl, Arizona Republican and Senate minority whip, posed 12 questions suggesting they had serious national security concerns about Russian involvement in missile defense cooperation.

They also asked for documents regarding the secret meetings on missile defense held between Mrs. Tausher and Russian Deputy Foreign Minister Sergei Rybakov, who have been leading the missile defense negotiations.

A senior State Department official said the proposed BMDC agreement is still being negotiated. “We are still talking,” said the official, who noted “enormous success” in talks with the Russians during the NATO summit.

“The Russians are interested” in cooperation with NATO, the official said.

In October, six Republican senators wrote to Mrs. Clinton asking whether any agreements with Russia will impose limits on defenses. The senators, led by Sen. Jeff Sessions of Alabama, stated that “senators must be assured that these talks and potential missile defense agreements will not limit U.S. and allied missile defense development and deployment in any fashion whatsoever.”

“We have stated, ad nauseam, we’re not limiting missile defenses,” the State Department official said.

Asked about Mr. Medvedev’s comments on the need for a missile defense agreement, the senior official said it was “a positive step” for the Russian leader to recognize the need to address common missile threats.

In Moscow, Mr. Medvedev said Russia faced two futures in the next 10 years: “Either we will reach an agreement on the missile defense systems and will create a full-fledged joint cooperation mechanism or, if we fail to reach a constructive agreement, a new spiral of the arms race will be launched.”

The Russian president said in a state-of-the-nation address to the Federal Assembly that the alternative is “a decision on the deployment of new offensive forces.”

Russia also is prepared to work toward countering the spread of missiles, noting that during the recent NATO summit in Lisbon, he proposed a possible European missile defense architecture that would link European and Russian defenses from missile attacks.

“We have already launched working on the whole scope of related issues. This is certainly the positive development,” Mr. Medvedev said.