The Soviet Union announced today that it was expelling Newsweek correspondent Andrew Nagorski, who was accused of using "impermissible journalistic methods."

It was the first such action against an American journalist in five years--only the third against a U.S. correspondent in more than a decade--and reflected the poor state of U.S.-Soviet relations.

Nagorski, 35, who has represented Newsweek here for the past 14 months, was summoned to the Foreign Ministry this morning. His accreditation was confiscated and he was told to leave the country as soon as possible.

Nagorski rejected the charges against him, which included posing as a Soviet journalist while visiting a provincial city, violating travel regulations for foreigners, and pretending to be a Polish tourist to gather information. The allegations date back to October last year, but he had received no previous warning from the Foreign Ministry.

The 30-strong corps of U.S. journalists based in Moscow have frequently been the target of harassment by Soviet security forces and allegations of spying or illegally gathering information. But expulsions have been relatively rare and have usually coincided with periods of diplomatic strain between Moscow and Washington.

The last expulsion of an American journalist from Moscow occurred in 1977 when a correspondent for The Associated Press, George Krimsky, was accused of espionage and violating Soviet currency regulations. He denied the charges, which were combined with a crackdown on contacts between Western journalists and Soviet dissidents.

In June of that year, Robert C. Toth of the Los Angeles Times was detained and interrogated at length by Soviet secret police on his sources for scientific reporting in an incident that drew strong expressions of concern from the White House and the State Department. He was allowed to leave the country at the scheduled end of his three-year tour a few days later.

A year later, Craig Whitney of The New York Times and Harold Piper of The Baltimore Sun were convicted of libel in a suit by the Soviet State Television and Radio Committee. The two correspondents in their dispatches had quoted dissident sources as saying that the televised confession of a Georgian dissident was faked. The dissident later said his confession was genuine. The two journalists refused to participate in the trial, although they did pay fines and court costs under protest, and they later were let off with a warning.

Earlier this year, a correspondent for the American Broadcasting Co., Anne Garrels, had her accreditation card confiscated after being involved in a traffic accident in which a man was killed. But she was not formally expelled from the Soviet Union.

The U.S. Embassy here was informed of the expulsion order against Nagorski this morning but declined comment until the State Department had an opportunity to study the case. In previous such incidents, U.S. administrations have applied the "reciprocity" principle and expelled a Soviet journalist from Washington in retaliation.

In Washington, a State Department spokesman said, "We would prefer not to comment until we have had a detailed report on the incident."

In New York, Newsweek Editor Lester Bernstein said in a statement that the charges against Nagorski "are totally untrue. He is a conscientious and responsible correspondent in whom we have complete faith."

Nagorski, who previously worked in Newsweek's offices in New York and Hong Kong, said he was told to report to the Foreign Ministry after returning to Moscow last Thursday from a trip to the town of Krasny Sever near the Soviet border with Afghanistan. While in the region, he and a reporter for the Chicago Tribune, Jim Gallagher, were accused of attempting to visit a border town called Kurgan Tyube, which is closed to foreigners.

The two journalists, however, said they had informed the Foreign Ministry in advance of their travel plans--as required by Soviet law--and had not received any objections.

Vast areas of the Soviet Union, particularly along sensitive borders, are out of bounds to Western visitors. Like other Westerners resident in Moscow, American journalists here are required to formally notify the Foreign Ministry whenever they travel outside the capital.

Nagorski, whose family is of Polish origin, said the charge of posing as a "Polish tourist" appeared to stem from a visit to the city of Rovno in June. Some local residents had mistaken his Russian accent for that of a Pole--but he said he had not attempted to hide the fact that he was an American reporter.

Announcing Nagorski's expulsion, the Soviet news agency Tass said he had posed as deputy editor of a newspaper in the town of Vologda by using the Soviet journalist's card. Nagorski's explanation for the October incident was that he had suggested to Soviet police that they ask the editor to vouch for him after stopping him for photographing street scenes.

"The allegation that I tried to pass myself off as a Russian journalist is ludicrous," he said.