Moscow has responded coolly to a U.S. invitation to monitor missile-defense flight tests.
Foreign Minister Sergei Lavrov in his remarks made clear the offer fell far short of Moscow's calls for a role in planning a missile shield and for a binding guarantee that the system would not weaken Russia.
Moscow says the system would weaken Russia if it's able to shoot down the nuclear missiles Moscow relies upon as a deterrent.
The United States has invited Russia to use its own equipment, including radar, to measure U.S. missile-defense tests.
The director of the Pentagon's Missile Defense Agency, Army Lieutenant General Patrick O'Reilly, said such action should convince Moscow the missiles are not powerful enough to threaten its own strategic deterrent missiles, which are based deep inside Russia.
O'Reilly said the SM-3 interceptor, to be based on land and sea, "can't reach that far."
Ellen Tauscher, undersecretary of state for arms control and international security, said the United States was prepared to offer Moscow written assurances that the system being built in Europe is not directed against Moscow.
But Taucher also added, "We cannot provide legally binding commitments, nor can we agree to limitations on missile defense, which must necessarily keep pace with the evolution of the threat."
Tauscher repeated Washington's desire to cooperate with Moscow on the project, but cautioned the opportunity to do so was not an "infinite opportunity."
Tauscher held talks on the topic with Russian Deputy Foreign Minister Sergei Ryabkov last week in Moscow.
Tauscher and O'Reilly were both speaking at a forum in Washington hosted by the Atlantic Council.
compiled from agency reports