A Soviet diplomat assigned to the United Nations mission in New York was detained yesterday by the Federal Bureau of Investigation, and administration officials said last night he will be expelled from the United States on grounds that he was engaged in espionage.

Administration sources identified the man as Mikhail Katkov, the second secretary of the Soviet mission.

The sources said Katkov was released after showing his diplomatic identification and the U.S. government is expected to expel him quickly. In cases like this, the sources said, the suspected spy generally is not arraigned in court.

Under usual procedures, the State Department would inform the Soviet Embassy of the incident and Katkov would be expelled from the country in the next couple of days.

Sue Schnitzer, an FBI spokeswoman, refused to confirm that Katkov had been detained. Justice Department spokesmen also refused to provide any details of the operation he was involved in, nor would they say where he was detained.

The incident, which comes just a week after President Reagan's summit here with Soviet leader Mikhail Gorbachev, was the first espionage incident involving the Soviets since Gennadi Zakharov, another Soviet official assigned to the United Nations, was arrested Aug. 23, 1986, on a Queens, N.Y., subway platform after he allegedly was caught accepting classified documents from a foreign student who was cooperating with the FBI.

Zakharov, the Soviet scientific attache, did not have diplomatic immunity and was kept in U.S. custody for arraignment. In retaliation, the Soviets picked up American reporter Nicholas Daniloff, accused him of spying and held him in a Soviet prison.

Daniloff, a reporter for U.S. News and World Report, was finally released Sept. 29, 1986, in an apparent deal between the two countries. A day later, Zakharov pleaded no contest to espionage charges in federal court in Brooklyn. U.S. District Judge Joseph M. McLaughlin sentenced him to five years probation outside the United States and ordered him to leave the country within 24 hours.