A fire broke out on the upper floors of the U.S. embassy in Moscow late today, threating secret documents and sophisticated radio equipment.

Soviet firemen and police sped to the scene and ordered the U.S. personnel guarding the documents to leave the building. United Press International reported that the Americans refused and stayed on in the smoke-filled area with the aid of oxygen masks.

U.S. Ambassador Malcolm Toon, watching the smoke billow from the upper-floor windows as he stood on the sidewalk dressed in a dinner jacket, said no injuries had been reported.

Ten truckloads of Soviet firemen responded to the alarm at 10:30 p.m. (3:30 p.m. EDT), raising ladders up the side of the building as crowds of onlookers gathered.

Although firemen declared the blaze under control shortly after midnight, the flames flared up again, Reuter news agency reported.

More than two hours after the fire started, flames were shooting out of the top half of the building, a hundred years from the Kalinin Prospekt, one of the city's main thoroughfares, Reuter said.

The affected areas of the building, containing sensitive documents, appeared to include Toon's office, the embassy's agricultural and public-affairs sections and possibly parts of the defense attache's office.

All these areas are designed as secure parts of the embassy, and Russians are not normally permitted inside. The entire building is considered U.S. territory under international law and Soviets may enter only with explicit American permission.

Soviet firemen appeared to be seeking permission from Toon for each of their actions:

The Americans were believed to be reluctant to let large numbers of Soviet firemen into sections of the building that could contain classified materials.

Agency reports conflicted concerning the efforts of U.S. Marine guards of battle the blaze.

United Press International said British embassy officials came to the aid of the Marines who remained on the upper floors, providing spare oxygen tanks after the Soviet firemen intially refused to provide tanks and ordered the Americans to leave.

The Marines and an embassy officer who stayed behind with them had only 10 minutes of oxygen left by the time the British spares arrived, embassy sources said, according to UPI.

But Reuter said the Marines, who first tackled the blaze before calling the Soviet fire department, went back into the building wearing Soviet oxygen equipment.

Embassy officials and their families, whose apartments are in the building from the seventh floor down, were evacured shortly after the blaze was reported. They stood in their night-clothes on the sidewalk in front of the 10-floor building on Chaikovskovo Boulevard in central Moscow, watching the firemen pour water on the flames.

Among the evacuees were children in bathrobes standing on the street as firemen entered the building through an upper-story window.

As Soviet firemen ran into the building wearing breathing apparatus, one American said:

"When I got out the flames were licking out of one room at one end of the floor, and it looked as though the fire caught most of the floor.