The Soviet Union has agreed for the first time to allow an American group monitor underground nuclear weapons tests with seismic equipment located inside Soviet borders, an official with the group, the Natural Resources Defense Council, said here yesterday.

The Soviets also reportedly have agreed for the first time to detonate a chemical explosion near their eastern Kazakhstan nuclear test site in an attempt to ensure the accuracy of the American monitoring effort, according to S. Jacob Scherr, the NRDC's senior staff attorney.

At the same time, in an apparent concession to their military, the Soviets insisted that three NRDC seismic monitoring stations now in the Soviet Union be moved further away from the test site.

"We are pleased that the Soviets are interested in continuing and expanding our existing nuclear test ban monitoring program," Scherr said in an interview late yesterday.

Scherr said the agreement was signed in Moscow yesterday by Adrian DeWind, chairman of NRDC's board, and by a senior official of the Soviet Academy of Sciences. Word of the pact came to Scherr in a telephone call from the group's senior scientist, Thomas Cochran, who is in Moscow with others involved in the project.

Under an agreement with the Soviet academy, NRDC has maintained three seismic stations roughly 120 miles from the Soviet test site since late last summer, but it has been told to shut the stations off each time the Soviets conducted a nuclear explosion. Since abandoning a unilateral nuclear test moratorium in February, the Soviets have set off nine nuclear explosions, including seven said by the Soviets to be weapons-related.

The new agreement will allow NRDC monitoring to continue during Soviet tests. But the equipment must be moved from its current locations to new sites at least 600 miles away from the Semipalatinsk test site.

The United States maintains a wordwide seismic network to monitor Soviet tests, but none of the stations is in Soviet territory.

The Soviets agreed to expand NRDC's sites from three to five, and establish a direct telephone line between U.S. and Soviet participants to exchange seismic data, Scherr said. The Soviets also agreed to detonate a large chemical explosion that would serve as a "calibration" for NRDC measurements.