Black nationalist guerrillas killed 10 of the 18 survivors of an Air Rhodesia plane that crashed in rugged northwestern Rhodesia Sunday night with 56 persons aboard, according to Rhodesian authorities.

A military communique issued here last night said that nine guerrillas reached the scene of the wreckage ahead of rescue workers and opened fire at point-blank range on 13 survivors who had remained by the plane while five others went for help.

It is the worst air disaster in the history of Air Rhodesia. This plus news of the gunning down of 10 of the survivors has dealt a further shock to the breakaway colony, which is already exhausted by war.

Among those killed by the guerrillas, the communique said, were six females, including two girls aged 4 and 7 and a woman who had torn pieces of cloth from her dress to administer first aid to another survivor.

The guerrillas were reported to have remained long enough to loot the charred remains of the plane before leaving.

The turbo-prop four-engine Viscount crashed shortly after taking off from the holiday lakeside resort town of Kariba, enroute to the capital. Most of the 52 passengers and all four crew members appear from the list of published names to have been whites, with the exception of eight members of an Indian family aboard the plane.

If the 10 survivors who are killed by the guerrillas prove to be all whites, it would be the second largest number of white civilians to die in a single incident since the guerrilla war began here five years ago.

On June 23, 13 British missionaries and their children were axed, clubbed and bayonetted to death at a mission in the eastern border area.

The cause of Sunday's crash is still not known, but Rhodesian government authorities so far are lending no credence to speculation that the plane may have been hit by a missile fired from the wooded terrain into which it plunged only five minutes after takeoff.

The guerrillas are thought to have some Soviet-built shoulder-fired missiles, and there have been several unconfirmed reports of Rhodesian civilian and military planes being fired upon.

A government spokesman said last night that so far the cause of the crash is believed to have been a possible power failure in its two starboard engines. The last words from the pilot in an emergency message were that he had lost both starboard engines and that "we're going in."

The military communique said it appeared that a starboard engine had exploded and that the side of the plane was heavily scorched. This led some observers to speculate the engine might have been hit by a missle.

The site of the crash was described as being 50 miles west of Karot, which lies between Lake Kariba and Salisbury. The region is known to be heavily populated with guerrillas, most of them belonging to Joshua Nkomo's wing of the Patriotic Front, the alliance of two black nationalist armies fighting to overthrow Rhodesia's present biracial government.

The hunt for the missing plane began almost immediately after the pilot radioed his SOS call late Sunday afternoon, but it was called off late that night. The plane finally was sighted at first light but it apparently took some time for the security forces to reach the scene.

The communique said that according to witnesses 18 of the passengers had survived the crash and were still alive as of 5:45 p.m. Sunday night. Of those, five had left through the thick bush to seek help from local tribespeople and the rest remained close to the aircraft it said.

"Terrorists later approached the scene and ordered the shocked and numbed survivors to their feet. The terrorists then opened fire with Communist-made Kalashnikov assault rifles and 10 of the passengers . . . died in a hail of fire," it said.

The remaining eight survivors have been taken to hospitals in Kariba and Karoi.

Reuter reported from Kariba:

The three survivors of the assault said the guerrillas at first told the injured they were going to give them food, water and help.

"They got us together, forcing those who could walk to carry those who couldn't," said Hans Hansen, 35. "Then they opened up with sustained automatic fire."

The three said they escaped by fleeing into the night when the firing began.

"It was the most brutal thing I have ever seen," said Hansen, who escaped with his wife Diana, 31, and Anthony Hill, 39.

Hansen said the guerrillas spoke English and told them: "You have taken our land. We are going to kill you all."

The three, interviewed at Kariba Hospital, said they were trying to make themselves comfortable for a night in the bush when the guerrillas arrived.

"When the terrorists opened fire we ran over a small ridge," said Hill. "We found a patch of grass in the bush and slept there for the night."