Libyan troops defending Kempla have been ordered to flee the besieged Ugandan capital for an air base 70 miles north, apparently for evacuation home, diplomatic sources said today.

Their departure, along with the reported fall of nearby Entebbe Airport and President Idi Amin's stately residence there, reinforced the growing impression that the fall of Kampala to Tanzanian troops backing an anti Amin revolt is imminent.

Kampala residents, reached by telephone, said the city was quiet and neither Ugandan nor Anzanian soldiers were to be seen on the streets. Libyan soldiers, who were visible manning antitank emplacements and artillery positions before the weekend, had vanished, they said.

Exile sources here expected Amin to leave with the Libyans from the Israeli-built air base at Nakasongola. Amin, however, had his radio issue an apparently fanciful war communique claiming that the Tanzanians at Entebbe and on the fringes of Kampala were surrounded.

The radio claim illustrated the difficulty of determining with precision how the assault was progressing, with reports from both sides designed for propaganda purposes and no correspondents in the Kampala area.

Heavy fighting on the road between Kampala and Entebbe was reported by a British family that fled to Kenya in a horrowing drive through artillery fire. The evacuees confirmed reports from Kampala that the airport and Entebbe town had fallen to Tanzanian troops and Ugandan exile forces.

This may have been the final battle of the war as the remnants of Amin's army who defended Entebbe did not regroup to defend Kampala, diplomats reported. One said the city was as ready to receive the Tanzanians as Saigon had been for the Communist Vietnamese in 1975.

"The war is over," he said.

At least half a dozen of Ugandan's ministers are in Nairobi. Some-such as Amin's close Nubian associate. Brig. Ismael Sebi, minister of animal resources; Maj. Bob Astles, Amin's British-born aide; and Brig. Ali Fadul, minister of provincial administration-have reportedly been trying unsuccessfully for several days to get Kenyan aid for Amin.

Others, who refuse to say whether they have defected or are still working for Amin, include the foreign minister, the housing minister, the governor of the central bank and a former representative to the United Nations.

One Ugandan still in Kampala said by telephone he and his friends were waiting for the Tanzanians and they were welcome "after eight years of waiting for someone to rid us of this scum."

Diplomats in the city reached their offices with no difficulty and denied earlier reports that the invaders had reached the landmark of the Queen Elizabeth clock tower. "I have walked there this morning and saw no one," said one diplomat.

Diplomats and exiles concurred that all roads out of Kampala are now controlled by the invading forces except the road east to Jinja and the Kenyan border.

Radio Uganda said Amin was still in the capital with his troops. But the radio is not considered a reliable source of information, having been used in the past to broadcast untrue messages from the brutal Ugandan dictator.

Amin was last seen in Kampala Wednesday night. The radio annouced that the president for life is "still going strong." But neither the Information Ministry nor the radio station answered its telephones.

Tanzania's Foreign Minister Ben Mkapa arrived in Nairobi today with a special message for President Daniel arap Moi form Tanzania's President Julius Nyerere. Kenya's relations with socialist Tanzania have been strained since the disentegration in recent years of the East African Community, which linked them and Uganda.

The Tanzaninas are reported eager to reassure the Kenyan government that the umbrella guerrilla movement they have backed, the Uganda National Liberation Front will not be a repeat of the socialist Milton Obote government, which preceded Amin and was unsympathetic to Kenyan commercial and trading efforts in central Africa through Uganda. CAPTION: Map, no caption By Dick Furno-The Washington Post Picture, Ugandan refugees gather at Kenyan town near border after fleeing the fighting. AP