The Natural Resource Defense Council, a private U.S. research organization, signed an unusual agreement last week with the Soviet Academy of Sciences that would provide joint monitoring of U.S. and Soviet underground nuclear tests, NRDC officials said yesterday.

Facilities in each nation would be jointly staffed and operated by scientists from the two organizations, and data developed by the two seismic stations would be delivered to both governments, according to Dr. Thomas B. Cochran, NRDC senior staff scientist who originated the idea.

"We will not referee whether a test has taken place or the size of the yield of any test," Cochran said. Instead, he said, the data "would be folded in with the rest of the data each government collects."

The NRDC's objective, Cochran said yesterday, is "to demonstrate in-country stations are not an obstacle to a test moratorium."

The State Department, which was aware of the negotiations, has not taken a position on the agreement signed in Moscow May 28. Implementing it requires U.S. approval for export of monitoring equipment and of visas for Soviet visitors.

Under the pact, the NRDC would establish three seismic-monitoring stations in the Soviet Union within 200 kilometers of its main underground nuclear-weapons test site near Semipalatinsk in the south-central U.S.S.R.

Three sites would also be located in the United States near the Department of Energy's Nevada Test Site.

Yesterday, Cochran and NRDC Chairman Adrian W. Dewind discussed the agreement with Deputy Secretary of State John C. Whitehead, who told them that the department has not formulated its position on the NRDC initiative.

The two nations have been at odds on the nuclear-testing issue.

Last July, Soviet leader Mikhail Gorbachev announced a five-month testing moratorium and asked the United States to join in an effort to end nuclear tests. The moratorium has been extended several times, most recently after the nuclear power plant accident near Chernobyl.

The Reagan administration has refused to end nuclear tests, and President Reagan has pressed Gorbachev to agree to on-site monitoring of tests to permit better verification of the signed but unratified Threshold Test Ban Treaty. That 1974 agreement limits both sides to tests of 100 or fewer kilotons.

Reagan has said that, if Moscow accepts on-site montoring, he would push for ratification of the treaty. He has refused to halt U.S. tests as long as he believes nuclear weapons are needed for deterring Soviet attacks.

Cochran said the agreement was signed by Dewind and E.P. Velikhov, vice president of the Soviet Academy and a key adviser to Gorbachev in scientific and nuclear affairs.

The NRDC must raise $500,000 to purchase seismic equipment and begin the project, Cochran said. NRDC spokesman said that some foundations have shown interest and that the group hopes to launch the program by July.

The NRDC spokesman said that if an export license for U.S. equipment is not forthcoming, the group could use Soviet equipment, which is not thought to be as good. "The U.S. interest is not to hinder the effort," the spokesman said.