The Soviet government today announced the expulsion of an American diplomat on charges of espionage and said he was caught "red-handed" with incriminating spy equipment three days ago.

The action against Richard Osborne, a first secretary at the U.S. Embassy since he arrived here last August, was the first known Soviet expulsion of a U.S. diplomat in Moscow in five years.

[The State Department confirmed that Osborne had been expelled but it and the White House refused to discuss the Soviet allegations against him.]

In announcing that Osborne had been declared persona non grata, the government news agency Tass said agents of the KGB, the Soviet secret police, seized the diplomat "while he was working with an espionage radio apparatus" and also confiscated other evidence that "exposed his espionage activities."

The radio equipment was described as "a set of portable intelligence special-purpose apparatus for the transmission of espionage information via the U.S. Marisat communication satellites."

Marisat is the Maritime Communications Satellite system, used for worldwide communications by the U.S. Navy and also commercial interests.

Tass said the agents also seized Osborne's "own notes which were written in a pad made of paper that quickly dissolves in water."

The announcement said Osborne was ordered to leave the country "for actions incompatible with diplomatic status."

The last U.S. diplomat known to have been expelled from the Soviet Union was Donald Kursch, a first secretary of the embassy, in January 1978. His expulsion was in retaliation for the expulsion from Washington of a Soviet trade official.

The last embassy employe ousted from Moscow on espionage charges was Martha Peterson, also allegedly caught "red-handed" by KGB agents, in 1977. Her expulsion was not disclosed by the Soviets until a year later, however.

The U.S. Embassy appeared to have been caught by surprise by the Tass announcement in mid-afternoon and refused to give any information about Osborne, including his age or home town.

Osborne's wife, Mary, a substitute teacher at the Anglo-American School here, seemed suprised by the news that was read to her over the telephone by a reporter. She started to sob and she appeared not to have been aware that her husband had been detained briefly by the Soviet police on Monday before being released. The Osbornes have two children, ages 6 and 8.

Osborne, a soft-spoken man in his mid-30s, is listed as working in the embassy's economic section. His duties are said to have involved Moscow's economic ties with the Third World.

Tass gave no additional details as to the circumstances surrounding the incident.

Standard practice in past cases of American diplomats accused of spying has been that their expulsions were treated discreetly. Usually, the Soviets have waited for several years before publishing accounts about their actions.

A year ago, the United States expelled Vasili Chitov, a senior Soviet military attache, from Washington on espionage charges. Moscow had not taken retaliatory steps in connection with that incident and it seemed unlikely that Osborne's ouster was in retaliation. The Soviets, when retaliating, normally expel a person of the same rank and do so within weeks.

Diplomats here speculated that the move against Osborne could signal a tougher Soviet attitude toward the Americans in Moscow. Other diplomats suggested that the Soviets felt the need to get tough following a series of expulsions by several West European countries of Soviet diplomats and other personnel in recent months.

A spokesman for the U.S. Embassy said Osborne and his family were preparing to leave Moscow shortly.