MOSCOW, FEB. 17 -- A fire at the U.S. Embassy burned for 45 minutes today before Moscow firefighters brought it under control.

The fire, on the fifth floor of the "old" embassy building, came in the midst of the staff's preparations for Secretary of State George P. Shultz's visit Sunday.

No one was hurt and the fire was limited to a single unoccupied room, one of many under renovation, in part for fire-prevention purposes. Embassy spokesman Richard Gilbert said the construction might be to blame but that "it will take a full investigation to be sure."

The old embassy, a yellow stucco building constructed in 1953, was called a "firetrap" by Rep. Dan Mica (D-Fla.) when he came here last April as part of a delegation investigating security concerns. A new building, which many U.S. officials believe was bugged by Soviet construction workers, is 200 yards away.

The fire began at 2:45 p.m. and smoke and flames could be seen spewing out the windows. At first, U.S. marines tried to bring it under control with extinguishers. Alarms were sounded.

Gilbert said there were about 150 people in the building when the fire began. He said that as soon as the alarms went off, "the staff locked up their papers and evacuated." Ambassador Jack F. Matlock Jr. was not in the building at the time.

After several minutes, three Soviet fire trucks arrived. American guards kept the firemen waiting for about five minutes at the entrance until U.S. officials could escort them.

Embassy staff members are still smarting from the controversy over security and they were quick to note that the fire never reached the nine-story building's top four floors, where the main offices are located.

In 1977, a much larger fire swept through three of the old embassy building's upper floors. There was concern then that Soviet firemen had been able to walk freely through secure areas.

"It was an ordinary household fire that represented no particular challenge from the professional point of view," said Gen. Viktor Kononov, supervisor of the fire-fighting service at Moscow's militia department.

In an interview with the official Soviet news agency Tass, Kononov noted that while the marines had tried to damp the fire, which enveloped tables, chairs and construction debris, their efforts were "to no avail."