The United States planted nuclear-powered devices on the ocean floor to monitor international telephone calls in and out of the Soviet Union, KGB chief Viktor Chebrikov was quoted as saying Friday. In an interview with the daily Pravda that offered a rare glimpse into Soviet counterintelligence operations, the KGB chief also said his agents have caught 20 spies in the last 21/2 years, some of them within the KGB's own ranks. Chebrikov, 65, a member of the Communist Party's ruling Politburo, made it clear that despite the warming trend in East-West relations, the superpower intelligence agencies remain vigilant. "Certain circles of imperialism have not abandoned the course toward confrontation," Chebrikov said. He said his agency discovered the nuclear-powered monitoring devices on an underwater telephone cable in the Okhotsk Sea 40 miles off the Soviet east coast and north of Japan. "Each container weighed six tons and was stuffed with American reconnaissance equipment," he said.