Arkady N. Shevchenko, a former senior adviser to Soviet Foreign Minister Andrei A. Gromyko who defected to the West in 1978, was a top CIA informant who provided the United States with crucial intelligence information for 32 months before he defected, according to excerpts from his memoirs published today in Time magazine.

When Shevchenko, 53, asked for asylum, he was undersecretary general of the United Nations and an arms control specialist, the highest-ranking diplomat to defect since World War II.

Reports circulated at the time that he had been in touch with U.S. intelligence agencies, but they were not confirmed.

In his new book, "Breaking With Moscow," being published later this month, Shevchenko says he first approached Americans about defecting in 1975 and was told to provide information first as a kind of sincerity test.

In an interview broadcast last night on CBS News' "60 Minutes," Shevchenko said the CIA made him keep working undercover longer than he wanted to.

"I never had an idea of a long period of spying but . . . what can I do, you know, and they could even betray me to the Soviets," he said. "I was actually in their hands."

Sen. Daniel Patrick Moynihan (D-N.Y.), who was U.S. ambassador to the United Nations at the time, told interviewer Mike Wallace that through Shevchenko, "for the first time we got an understanding of how Soviet foreign policy is made and how it is operating . . . . It was invaluable. Nothing like it had ever before occurred."

Shevchenko said in the interview he had provided information on Soviet intentions in Africa and Central America and on Politburo debates over China. "There was a period when the Soviet Union . . . was really considering the idea of using nuclear weapons against China," he said.

Gromyko is "half human, half a machine, or a computer," he said. Other leaders "live in almost complete isolation from the ordinary people," but they "don't intend to use nuclear warfare weapons against the United States. I'm sure of it," Shevchenko said.

Moynihan said he helped conceal Shevchenko's role by "being as disagreeable in public as I possibly could" to him.

His CIA handlers took extraordinary security measures, renting an apartment in Shevchenko's building to facilitate keeping in touch.

When Kremlin leaders began to suspect Shevchenko's double life, he defected so abruptly he almost abandoned his wife, who knew nothing. Soviet officials said she later committed suicide.

Shevchenko subsequently had a very well-publicized relationship with Judy Chavez, a self-described call girl he met through the FBI. Their expensive travels and parties made the news and embarrassed the CIA.

"I was stupid enough to pay [her] quite a substantial amount of money," Shevchenko said. He later married a U.S. citizen, Elaine Jackson