The Soviet Union, in an unusual display of openness, has agreed to allow U.S. researchers to see "oceans" of archives that one historian says show incidences of collaboration with Nazi occupation forces in World War II.

After a preliminary survey of the material this month, "our historians tell us they have seen documents that have not been seen by Western eyes for the past 40 years," said Miles Lerman, chairman of the Committee of International Relations for the United States Holocaust Memorial Council.Lerman signed an agreement July 29 in Moscow which will allow U.S. historians to photograph the documents for storage in the Holocaust museum Congress has authorized the council to build on the Washington Mall by 1991.

The Soviets also will receive microfilm and microfiche copies of the documents, made at the expense of the U.S. council.

"By microfilming and microfiching the documents we extend their shelf life by at least 300 years," said Lerman. "That's one of the things that turned them around."

The Kremlin has long blocked Western and Soviet historians from viewing archives, including those from the period of Nazi occupation, and U.S. historians attribute the shift to the reform policies of Mikhail S. Gorbachev.

"This is a 180-degree turnaround in the attitude of the Soviet authorities," said Raul Hilberg, a University of Vermont historian who accompanied Lerman on his trip to Moscow and reviewed documents in the Soviet capital and other cities.

"It's a humongous amount, oceans and oceans of material" in three dozen archives administered by Glav Archiv for the Soviet Council of Ministers, said Lerman.

The archives will not be opened to Soviets, said Lerman and Hilberg.

In the past, the Soviets have given the West piecemeal archival materials, such as the identification card that was crucial in the conviction last April of John Demjanjuk. He was sentenced to death in Israel for operating gas chambers at Treblinka, the Nazi camp in Poland where hundreds of thousands died.

Hilberg, who has been studying Holocaust documents for 40 years, said the Soviet archives provide extraordinary detail of the day-to-day operation of Nazi death camps and the security forces under German occupation.

"The collaboration was much greater in areas held by the Soviet Union for brief periods of time" before the war, "higher in the west Ukraine than in the east, higher in the Baltic buffer states and eastern Poland," said Hilberg.

Those areas were annexed by the Soviet Union before 1939.