The Soviet government news agency Tass said tonight that a nuclear powered satellite reported by U.S. military experts to be out of control had terminated its mission on Dec. 28 and that steps had been taken to ensure that its nuclear pack would burn up during its re-entry into the atmosphere.

Quoting "competent Soviet organizations," Tass said Cosmos 1402 was "divided into separate fragments by commands from Earth in order to isolate the active part of the reactor, which ensured its subsequent complete combustion in the atmospheric strata."

Tonight's statement appeared to contradict a statement yesterday by Vladimir Kotelnikov, a member of the Soviet Academy of Sciences, who told a press conference here that the craft was safe and still working normally.

The phrasing of the Tass report suggested that some parts of Cosmos 1402 may still be in orbit; it emphasized that only the nuclear power pack was ordered back into the atmosphere where it was to burst into flames and disintegrate. "The radioactivity level will remain within natural background limits," the report said.

The State Department said it had no comment on the Tass statement. A Pentagon spokesman said that the United States was aware that the Soviet satellite had disintegrated, but he said that "three fairly large pieces" were "still orbiting" and that "we believe the nuclear reactor to be still in orbit."

Before Tass published the new Soviet account, a State Department spokesman said Richard Burt, the acting assistant secretary of state for European affairs, had asked for a meeting later yesterday with Soviet Embassy officials to discuss possible dangers posed by the satellite.

The State Department spokesman said the United States also was setting up a "watch committee" of representatives from countries under the satellite's orbit and had told its embassies abroad to offer U.S. assistance in dealing with radiation problems if radioactive portions of the satellite should reach the ground.

Tass today described the craft as one involved in the "continued research of outer space," the standard phrase the Soviets use for all vehicles in the Cosmos series.

U.S. experts believe the craft was a spy satellite, whose job was to orbit the earth about 150 miles above it and keep track of the movements of American warships, including deeply submerged submarines.

The State Department said yesterday that U.S. specialists believe the craft was out of control and was likely to crash land on Earth by the end of this month. A spokesman said U.S. officials had been in touch with the Soviet authorities and have been told that the craft was not expected to drop out of orbit.

Tonight's Tass dispatch made no reference to what is normally included about guided re-entries of Soviet spacecraft and western observers saw this as suggesting an unexpected destruction of the craft before the completion of its mission.

The report was also seen as an attempt to calm fears abroad that the vehicle could crash land in the way a Soviet satellite crashed in Canada's Northwest Territories in February 1978.

The radioactive debris of that satellite rained across a 250-mile stretch in the empty pinelands around Canada's Great Slave Lake. However, none of the sample fragments picked up in the area was radioactive.