U.S. embassy officials reported today that a preliminary inspection by experts indicates that their chancery is in no danger of structural collapse from the effects of a weekend fire that burned for 18 hours in the top floors before being extinguished.

However, they said, it will be another day or two before the comprehensive report of the fire damage is completed by a team of engineers flown here from Washington to assess the calamity.

One source said that the interior of the eighth floor, where the blaze is believed to have started, must be completely rebuilt, including some sections of the floor. A permanent new roof will also be built to replace the old one, which was virtually destroyed by flames that burst through it early Saturday morning only minutes after Soviet fire officials had told the Americans they were sure they had controlled the fire.

At one point during the blaze, which began Friday night and continued through Saturday afternoon, American security guards guiding Soviet firemen inside the building Soviet firemen inside the building to keep them from breaching its security were withdrawn when Ambassador Malcolm Toon was told the entire central segment of the embassy might collapse. Firemen were left for some minutes by themselves in areas containing sensitive classified materials from which Soviets normally are barred.

Toon has said that he believes that no breach of security occurred and that the fire itself was caused by faulty wiring and not "sinister Soviet" deeds. He has estimated that the firemen may have been alone in the building for no more than 20 minutes, although a chronology of events of the fire he recited at a press conference indicates that it could have been an hour or more.

Seasoned observes here have long been impressed by the zeal with which the embassy staff takes its security duties. There are locks and combinations throughout the four working floors of the 10-story building, and visitors are escorted to and from offices.

The embassy's vital radio communications with the United States and the other U.S. diplomatic posts around the world have been restored by installation of emergency equipment.

Officials were finding replacement quarters for the gutted eights floor and damaged ninth floor by crowding into undamaged offices on the embassy first floor and across the street in a building housing the Commerce Department's office here. It was understood that the military attaches are again at work in their 10th-floor offices, from which Toon repeatedly barred Soviet firemen despite warnings that they need to use that floor to fight the fire effectively.

Today, Toon was back at work in his own 9th-floor office, which was left relatively untouched by the fire.