No one is shocked that spies might be in Russia’s delegation, and shrinking its size would hardly eliminate the risk of espionage, said the Western officials, who asked not to be identified talking about intelligence issues. But they said it could make the problem easier to manage.

Besides, now that cooperation between Russia and NATO has been suspended, the Russian delegation has far more people than it could possibly need, the officials said.

Well staffed though it may be, the Russian Mission to NATO has not rushed to comment on the decision. A spokeswoman for the mission said Thursday that Alexander V. Grushko, Russia’s ambassador to NATO, might speak, but not before Friday morning.

The move is only the latest difficulty in a relationship that has been fraught from the start.

President Vladimir V. Putin of Russia has objected strenuously to the alliance’s decision to take in new Central European members and expand to include 28 nations. He has been equally vociferous about NATO’s plans to field antimissile defenses in Europe.

The United States maintains that the antimissile systems are directed against a potential Iranian threat, but Russia fears they may eventually evolve into a system that could blunt Russia’s nuclear deterrent.