The Soviet Union, apparently eager to rid itself of seven anti-whaling activists who invaded Siberia by sea Monday, said yesterday that it will release the six Americans and one Canadian, and that all are in good health.

Officials of Greenpeace, the anti-whaling group that sponsored the mission, said they have asked American authorities to allow their ship, the Rainbow Warrior, to sail back to Siberia to retrieve its lost crew members.

French news reports quoted sources as saying that the Soviets had insisted that the seven be handed over to U.S. and Canadian authorities outside Soviet territory, probably aboard an American ship.

The Soviet Foreign Ministry told Canadian and American authorities in Moscow that the seven were "detained--not arrested--and they are going to work with the United States and Canadian authorities to arrange the eventual transfer of those individuals back home," said Marc Lortie, press attache for the Canadian Embassy here.

"As to how and when, whether it's a day, a week or several weeks, those details remain to be worked out," Lortie said.

State Department spokesmen said the Soviets reported that all seven are "in good condition."

The seven men and women belong to Greenpeace, an international pacifist and environmental group that has protested Soviet whaling practices since 1975. They and 15 other Greenpeace members sailed across the Bering Strait and into Soviet waters Monday to photograph a Siberian whale-processing plant on the Chuckchi Peninsula and publicize their cause.

They said Soviet soldiers detained six of them--including Greenpeace USA director Chris Cook, 30, of Washington--in the Siberian village of Lorino as they distributed Russian-language leaflets denouncing the Soviets for killing whales. A seventh was plucked from a motorboat by a Soviet military helicopter as he tried to escape, they said.

The other 15 arrived in Nome, Alaska, early yesterday with tales and photographs of a dramatic, zig-zagging sea chase across the Bering Strait, claiming that a Soviet vessel pursued the Rainbow Warrior for more than an hour.

A State Department spokesmansaid the Soviets told U.S. authorities that the seven will be released "as soon as appropriate arrangements are made." Asked for details of those arrangements, the State Department had no comment.

Observers expressed some surprise that the Soviets agreed to release the invaders without pressing charges, since they apparently crossed the Soviet border without authorization.

As one American official put it, "I doubt they had visas."

Greenpeace spokesmen said that the Soviets would only draw more attention to their whaling practices by further detaining the seven.

"They're setting themselves up for a lot more political pressure that they don't need," a Greenpeace spokesman said. "We're a burr in the bear's back. They've got a thousand other issues that are a lot more important to them."

The 15 Greenpeace members who returned from the voyage claimed that the Soviets were using grey whales to feed mink on a fur ranch near the Lorino processing plant. The International Whaling Commission has asked that the Soviets kill grey whales only for "aboriginal consumption"--as subsistence for natives.

The commission voted a year ago, with strong U.S. support, to impose a worldwide ban on the commercial hunting of whales in 1986. Japan, the Soviet Union, Norway and Peru filed formal objections to the ban, which under commission rules allows them to ignore it.

The commission convened its annual meeting in Brighton, England, this week, and Greenpeace members said the Siberian mission was timed to coincide with that event.

Besides Cook, the other six in Soviet custody are Jim Henry of Orrs Island, Maine; Pat Herron, Seattle; Nancy Foote, San Francisco; Barbara Higgins, Philadelphia; Ron Precious, Vancouver, British Columbia; and Dave Reinhart, Albany, Ore.