The Central Intelligence Agency yesterday denied reports that cast doubts on the authenticity of Soviet defector Arkady Shevchenko's bestselling memoirs, "Breaking With Moscow."

Stories published in The Washington Post and The New Republic quoted intelligence sources who said that Shevchenko's role as an informant while working in the United Nations before his defection in 1978 was of limited value. They also said that Shevchenko wrote the book with the aid not only of friends and editors but of the CIA.

"Arkady Shevchenko provided invaluable intelligence to the United States government," the agency said in a statement. "CIA had nothing to do with writing his book."

The agency would not comment on details of either the June 6 story in the Post or the article by Edward Jay Epstein that appears in the current issue of The New Republic.

Shevchenko is out of the country and was unavailable for comment.

Officials for Alfred A. Knopf, which published the book, and Time magazine, which printed two excerpts, restated their support of Shevchenko's facts in "Breaking With Moscow."

Shevchenko's success as an author has helped him become something of a media star. He currently charges at least $10,000 for a lecture appearance and movie studios are bidding for rights to "Breaking With Moscow."