Joshua Nkomo, coleader of the black nationalist Patriotic Front, said yesterday in Lusaka that his guerrillas were responsible for shooting down a Rhodesian civilian passenger plane with 56 persons abroad.

He denied, however, that they had anything to do with the brutal slaying of 10 of the 18 survivors of the Sunday crash and said the Viscount turbo-prop was thought to be carrying troops and war material.

Here in Salisbury, government authorities sharply took issue with the Nkomo statement in an apparent attempt to calm the bitter and jittery white population and prevent the total collaspe of Rhodesia's already faltering tourist industry.

If the plane was brought down by guerrilla fire, it would be the first such incident in the history of civil aviation here and another indication of the escalating nature of the war. The incident could also seriously harm efforts to bring Nkomo into the transitional government now ruling Rhodesia.

"There is at present no evidence to suggest that the aircraft was brought down by hostile action," Capt. Patrick Travers, general manager of Air Rhodesia, told a press conference. "In our opinion, had the aircraft been hit by a crew's first reaction would have been to say so.

But he admitted it was "very, very unusual" for two engines to fail on the same side at once as the pilot, John Hood, radioed was happening minutes before the plane crashed into guerilla-populated terrain in north-western Rhodesia.

He also flatly denied that Air Rhodesia Viscounts had ever been used for military purposes at any time during the war.

Meanwhile, the first account of one of the survivors of the crash seemed to confirm the Nkomo claim that guerillas brought the plane down. The survivor, Anthony Hill, said from his hospital bed in Kariba that there had been a big explosion and "the whole plane shook" as flames poured from the starboard engines.

But Hill said in a television interview he could not state for certain that the plane had been hit by a missile.

The incident, with a distinct possiblity now that the plane was shot down by a Soviet-made SA-7 or Strela missile, has had an enormous impact on the 230,000 whites here, many of whom have taken to flying about the country because the roads have become unsafe as a result of guerilla ambushes.

"People are really bitter now," said one white government secretary. "They want to shoot and kill in revenge for what has happened."

The Government has mounted what Air Marshal M. J. McLaren, deputy commander of combined operations, called a "full-scale manhunt" for the guerillas who allegedly killed the survivors, and many off-duty members of the security forces are reported to have joined of the hunt.

"The murder of innocent civillians who had already survived a major air disaster must rank as a crime against humanity," the air marshal said, promising that the guerillas responsible would be hunted down and "made to pay the supreme penalty for this barbaric action."

he fact that Nkomo took responsibility for the shooting down of the plane also seems likely to have considerable impact on how whites here react now to Prime Minister Ian Smith's effort to bring the guerrillas leader into the teetering trasitional government.

Nkomo disclosed last weekend that he had held a secret meeting with Smith in Lusaka, Zambia, in mid-August to discuss the possible participation of the Patriotic Front in the present interim multiracial government. Smith was reported to have offered Nkomo leadership of the ruling Executive Council, a position that would set him up to become the first black leader of Zimbabwe (the nationalist name for Rhodesia) early next year.

Nkomo's statement accepting responsibility for shooting down the plane, said one Rhodesian government spokesman "is an embarrassment to Britain, the United States, and to Smith as well. How can anyone look at him as a reasonable man after this," he added, referring to persistent reports that both the British and American governments have been involved in secret efforts to get Nkomo lined up as the future president of Zimbabwe.

In interviews with the British Broadcasting Company and Reuter news agency, Nkomo said yesterday from Lusaka that "we brought that plane down but it is not true that we killed any survivors."

"The Rhodesians have been ferrying military personnel and equipment in Viscounts and we had no reason to believe that this was anything different," he said.

He said the plane, a regularly scheduled flight from the Lakeside holiday resort of Kariba to Salisbury, was flying over an "operation zone" of his guerrilla army when it was hit. He would not say what weapon was used to bring it down.

Nkomo's guerrillas are mostly Soviet-equipped and Cuban-trained and are believed to have a few of either the longer-ranged Strela heat-seeking missiles or the portable SA-7s, Travers said, however, there was no confirmation that any civilian Rhodesian aircraft had ever been shot at by a missile so far in the war.

What has upset whites here even more than the shooting down of the plane is the way in which 10 of the 18 survivors allegedly was brutally slain at the site of the crash. Travers said the 10 had been "bludgeoned, shot and bayonetted to death by a gang of unspeakable thugs."

Hill, one of three persons to escape death at the hands of whoever opened fire on 10 of the survivors, said they had showed up shortly after the plane crashed and at first seemed ready to help the survivors.

But after telling the survivors to gather at one point, one of the men, Hill said, cried out, "You have taken our land" and then they all opened fire from a distance of about 15 yards.

Five of the 18 survivors had already left the site to seek help from nearby villagers and Hill said he and another couple avoided being killed by running into the nearby bush and hiding until the men finished looting the plane and left.

Nkomo denied that his guerrillas had any interest in killing civilians and in his BBC interview called eyewitness accounts to the contrary "nonsense."

He insisted that all of the 48 victims had been killed in the crash. "If people had come with AKs (assault rifles), how would there have been three survivors? It doesn't make sense, does it," he told Reuter. "It's just that the regime does not want to admit that we brought the plane down."

He also decried the "massive outcry" in the West over the death of the plane's mostly white passengers saying, "You forget that the regime kills 30 of our people a day. So the life of a black person is different from a white person. A European child is supposed to be worth a million blacks."

"So far as we are concerned we are bringing down an aircraft that's being used to ferry military personnel and equipment," he added.

At his press conference, Travers said Nkomo's claim the Viscount was being used for military purposes "is a downright deliberate lie." He said Air Rhodesia was not engaged in any military operations and was not engaged in any military operations and had never been chartered or commandeered either to carry troops or military equipment.

The high probability that the plane was shot down raises major questions for the safety of civil aviation now in Rhodesia. Travers said that all flights were operating normally and on scheduled and that planes already had been taking certain precautions because of the general security situation.

He said it was the first air accident for Air Rhodesia in its 11 years of existence and suggested that the cause might have been "engine seizure," a fule leak, or a runaway prop, any of which could cause an explosion such as the survivors said they heard.

But no Rhodesian white is likely to believe this version of what happened, and it seems the downing of the Viscount indeed marks a new turning point in the general deterioration of the security situation, of the whites' confidence in air travel and perhaps, too, in the nature of warfare here.

"We have said that it (the war) is going to be intensified every day, we will make it much more bitter," said Nkomo.