U.S. officials said Friday they are monitoring a nuclear-powered Soviet satellite that the Tass news agency says will fall to earth this August or September.

Maj. Alex Mondragon, a spokesman for the U.S. Space Command, in Colorado Springs, Colo., said that the satellite, Kosmos 1900, "has been in a steadily decaying orbit for the last month. If nothing is done to correct its orbit, it will decay," and re-enter Earth's atmosphere.The official Soviet Tass news agency reported Friday morning that Kosmos 1900 has a nuclear power plant aboard and would fall to Earth in August or September but that the radioactive material aboard would not present a danger.

Neither Tass nor the U.S. spokesman described the mission of Kosmos 1900, but it is believed to be one of two RORSAT satellites the Soviets have in orbit to monitor the movement of American warships. RORSAT is an American designation which stands for Radar Ocean Reconnaissance Satellite.

The Tass announcement, made available by the agency's office in Washington, said that "the artificial Earth satellite Kosmos 1900, with a nuclear power plant aboard, was launched Dec. 12, 1987."

"According to competent Soviet oranizations, radio contact with the satellite was lost in April 1988. The satellite is continuing in its oriented flight, and the main service systems are functioning according to the program.

"The satellite will fly in orbit until August-September 1988, after which it will cease to exist. The satellite Kosmos 1900 has systems insuring radiation safety on completion of the flight," Tass said.

The Tass announcement came as a top Soviet space official, Roald Sagdeyev, and private American scientists urged a ban on nuclear reactors in earth orbit, a measure designed to block the Reagan administration's Star Wars program. Such a ban also would end RORSAT.

Sagdeyev, director of the Soviet Space Research Institute, told a news conference that he had been in the United States for several weeks and had no fresh information on Kosmos 1900.

The main target of the proposed ban on orbiting reactors, Sagdeyev and the Americans said, is the U.S. SP-100 program, a pilot for the orbiting nuclear reactors that many scientists believe would be needed to power the exotic weapons envisioned in President Reagan's plans for a space-based missile defense.

The proposed ban would not apply to nuclear reactors aboard missions in deep space, such as a manned mission to Mars, Sagdeyev said.

But "an agreement to ban nuclear reactors from orbit would be a major barrier to any future arms race in space since nuclear reactors are compact sources of large quantities of power necessary for many military purposes," said a joint statement signed by Sagdeyev and Frank von Hip-pel of the Federation of American Scientists.

Sagdeyev, an adviser to Soviet leader Mikhail S. Gorbachev, has been at the forefront of Soviet efforts to block the Strategic Defense Initiative, as "Star Wars" is formally known.

Some administration officials also have credited Sagdeyev and Yevgeny Velikhov, vice president of the Soviet Academy of Scientists, with bringing to Gorbachev's attention many U.S. scientific papers arguing that "Star Wars" may be an impossible dream. That perception is thought to have led Gorbachev to soften his anti-SDI rhetoric at the Washington summit last December.