Russia’s spy hunters are increasingly worried about China’s espionage, even as the two countries grow closer.

In public, President Vladimir V. Putin of Russia says his country’s growing friendship with China is unshakable — a strategic military and economic collaboration that has entered a golden era.

But in the corridors of Lubyanka, the headquarters of Russia’s domestic security agency, known as the F.S.B., a secretive intelligence unit refers to the Chinese as “the enemy.”

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