A team of Soviet scientists is planning to open a monitoring site for U.S. nuclear tests in Nevada later this year, American and Soviet scientists announced here today. The new site is a response to last week's start-up of a privately organized U.S. nuclear monitoring station near Karkaralinsk, Kazakstan, and will give the Soviet Union the capability to review U.S. nuclear tests on American soil for the first time.

The Natural Resources Defense Council, with offices in Washington and New York, and the Soviet Academy of Sciences signed an agreement last May to send teams of scientists to one another's countries to oversee nuclear testing. Their aim is to demonstrate that a nuclear test ban is verifiable.

But some of the U.S. environmentalists who initiated the unprecedented exchange of nuclear monitors have cast doubts on whether Moscow will follow through on its end of the deal.

"There is an indication of a lack of interest on the part of some of the Soviet scientists," said Thomas Cochran, senior scientist at the Natural Resources Defense Council and originator of the exchange.

"The purpose of the thing is for verification, and verification of U.S. tests is not an issue for the Soviets," he said in an interview.

Soviet scientists are more interested in analyzing earthquake data than in measuring U.S. tests, he said, adding that development of a seismological research program to review such data is still tentative.

But Yevgeny Velikhov, vice president of the Soviet Academy of Sciences, dismissed doubts about Moscow's commitment to the project, which were raised in a press conference here today. He confirmed that the Soviet mission will take place, adding, "Our main task is to monitor a complete end to nuclear tests."

Cochran said he expects that the Soviets will travel to Nevada in November to begin installing seismic monitoring equipment for the tests.

Velikhov, Cochran and other Soviet and foreign proponents of a ban on nuclear tests held the press conference at the close of a meeting of the International Forum of Scientists for a Nuclear Test Ban. They presented a unanimous declaration supporting the ban, passed by the 200 participants, who included a small group of American antinuclear activists -- including Daniel Ellsberg and representatives of Greenpeace.

They called on Moscow to continue its ban on testing, now 10 months old, and for Washington to join it, which it has refused to do, partially on the grounds that a test ban is not verifiable. Soviet leader Mikhail Gorbachev received a contingent from the conference at the Kremlin today and told them that their proposal "will be examined most closely. The Soviet government will take a decision and it will be conveyed to you."

The U.S. monitoring base opened last week in Kazakstan, and will continue operating for a year. It will be permanently staffed by two American specialists, who will rotate every two months.

The funding for the sites and equipment, totalling $1.3 million, will be raised by private American foundations and individuals.