A small West German plane crossed the Norwegian border into Soviet airspace a year after the dramatic flight by Mathias Rust to Moscow's Red Square, a Foreign Ministry spokesman said Tuesday.

Spokesman Lasse Seim said the Cessna 150 was on its way from Ivalo in northern Finland to Kirkenes in the northeast corner of Norway on Saturday when the pilot strayed over Soviet territory.The plane crossed deeper into Soviet airspace when it returned to Ivalo on Sunday, Seim said. He said the aircraft penetrated "several miles" over Soviet territory and may have violated Soviet airspace for 20 or 30 minutes.

Norwegian government sources and the West German Embassy identified the pilot as Andreas Sommer of Hamburg. No further information on him was immediately available.

In Helsinki, Matti Autio, spokesman for the Finnish Border Patrol, said Sommer was questioned and released, and left Finland Tuesday, apparently for Sweden.

Another Border Patrol official, reached by telephone at the northern base of Rovaniemi, said Sommer claimed he lost his way due to bad weather. The official, who spoke on condition of anonymity, confirmed it was cloudy in the area during the weekend.

A Soviet Embassy spokesman was quoted on Swedish radio as calling the incident "a stupid practical joke" rather than a provocation.

"No one wants to blow up this case," Seim said.

Seim said Norway contacted Soviet authorities on Sunday, and "we have the impression that they followed the plane. They knew about it."

The Aftenposten newspaper speculated that the Soviet may have refrained from shooting at the plane because of the superpower summit in Moscow.

Seim said the Soviet Border Commission asked for a meeting with the Norwegians on Monday, when they handed over a formal protest.

Asked if Saturday's flight was timed to the anniversary of Rust's flight, Seim said, "We don't know. But it seems likely, doesn't it?"

Rust, also of Hamburg, flew a similar Cessna to Moscow from Helsinki, Finland, on May 28, 1987, landing outside the Kremlin's walls. He is serving a 5-year prison term in the Soviet Union.

Rust was 19 at the time he made the flight. On Sept. 4, 1987, he was found guilty by a Moscow court of illegal entry into the Soviet Union, violating international flight regulations and malicious hooliganism. He was sentenced to 4 years in a labor camp but is currently being held in a Moscow prison.