MOSCOW (AP) _ Russia's coast guard seized a Greenpeace ship on Monday as the group was trying to investigate nuclear waste sites in Arctic waters, but the environmental activists claimed a moral victory.

In France, another Greenpeace ship was seized by naval police in Cherbourg and four crew members were arrested. They were protesting the imminent arrival of a Japanese freighter to pick up a shipment of plutonium.

The Arctic incident was the second in two years in which a Greenpeace ship was seized while trying to document nuclear waste dumping off Russia's north coast.

''Our goal is to make it clear that this environmental situation must be dealt with - and won't go away just because a Greenpeace vessel is removed from the Kara Sea,'' Greenpeace spokeswoman Eleanor O'Hanlon said.

She said the crew of the Greenpeace ship Solo had been put under ''official arrest'' in Russia, although it was unclear what that meant. She said Greenpeace expected the coast guard to tow the vessel from the area.

The radio and engine rooms on the 219-foot, Dutch-built tugboat have been sealed and both engines shut down, she said.

The coast guard stopped and boarded the Greenpeace ship Monday after it crossed about a mile into Russian waters and activists began measuring for radiation near the southern tip of Novaya Zemlya island, a former Soviet nuclear dump site and testing ground.

The Solo's crew at first ignored commands to stop, then obeyed after the coast guard vessel Ural fired warning shots, the coast guard said.

''They fired three warning shots across our stern,'' Greenpeace activist Thomas Schultz told The Associated Press by radio telephone from the Solo.

Among the Solo's 34 passengers was Russian legislator Anatoly Mostovoy, O'Hanlon said.

The activists wanted to investigate the alleged 1982 disposal of a derelict Soviet nuclear submarine, the K-27, which reportedly was sunk off the coast of Novaya Zemlya.

A year ago, Russian legislator Andrei Zolotkov disclosed that between 1963 and 1986, the Soviet Union dumped radioactive waste in leaky containers into shallow waters in the Barents and Kara seas.

But in February, Adm. Vitaly Zaitsev, then-commander of the Commonwealth naval forces, denied the military had dumped any nuclear waste at sea or on land around the island in recent years. Only low-level waste in tightly sealed containers was dumped there before 1985, he said.

Meanwhile, French police said Monday the four activists there were released after questioning and the ship, the Beluga, taken back to a civilian harbor.

Greenpeace said the Beluga was seized after entering the restricted military port at Cherbourg and anchoring next to the dock where the Akatsuki Maru is to be loaded with its cargo of plutonium.

The Akatsuki Maru, which left Japan on Aug. 24, is expected to arrive in Cherbourg soon to load 1 ton of plutonium, one of the most toxic substances in the world.

Japan has refused to specify the route the freighter will take back to Japan, where the plutonium is to be used as fuel in fast-breeder nuclear reactors.