Rhodesia said today that Zambian-based black nationalists shot down a Rhodesian civilian airliner yesterday and vowed to retaliate against both the guerrillas and neighboring Zambia.

Joshua Nkomo, leader of the Accused guerrillas, all but confirmed responsibility for his Zimbabwe African Peoples Organization, saying during a visit to Nairobi, Kenya, that if the plane was shot down "it can only have been our chaps" who did it.

Zambia braced itself for renewed Rhodesian ground and air attacks that together with the air crash seemed destined to dash any meager remaining hopes for a negotiated settlement to the worsening Rhodesian conflict.

The crash, the second in five months due to guerrilla fire, killed all 59 passengers and crew members aboard. The last incident, in September, led to a sharp escalation in the war, with Rhodesia attacking guerrilla bases in Zambia for the first time.

Rhodesian Prime Minister Ian Smith, saying it seemed clear that the Air Rhodesia Viscount plane had been shot down, indirectly blamed the United States and Britain. He cited the failure of President Carter and Prime Minister James Callaghan to mediate a peaceful resolution to the six-year-old guerrilla war, saying it was in their power to do so.

"Rhodesians and Zimbabweans say to them: why are you holding back? What more do you want from us?" he asked, referring to U.S. and British refusal to recognize his white-led biracial government.

Smith has been seeking such recognition since he reached agreement with three moderate black leaders last March and has mounted a campaign in Washington for congressional support.

State Department spokesman Hodding Carter sharply criticized the attack saying, "We cannot state too strongly how deeply we deplore and condemn this latest unwarranted act of violence against innocent civilians in Rhodesia," and added, "There can be no justification whatsoever for such actions."

In London, the British Foreign Office said the government was "horrified by this senseless act."

Rhodesian military sources said they thought the aircraft had been hit by two ground-to-air missiles similar to the way the first plane crashed in September, resulting in the death of 48 persons.

Nkomo said in Nairobi that he had not been able to confirm details of the incident with his headquarters in Lusaka where a spokesman for the group said there would be no official comment until Nkomo's return.

Nkomo said the attack was probably intended for a plane that left Kariba airport 15 minutes later carrying the top Rhodesian commander, Lt. Gen. Peter Walls.

"He must have changed planes so he is responsible for the deaths of all these other people because he is the biggest military target," he said.

The crash occurred about 28 miles due east of Kariba and only about 30 miles away from the scene of the first one last September. Both are located in areas heavily infiltrated by Nkomo guerrilla.

Meanwhile, black and white officials of the Rhodesian government were promising to retaliate vigorously in revenge for the downing of the airliner. One black minister, James Chikerema, a former guerilla leader himself, warned that "Nkomo should not weep when we retaliate and you can rest assured that we will retaliate."

Chikerema's white co-minister of transport William Irvine, said at a press conference in Salisbury that the plane had plunged vertically into the ground and that the wreckage was squashed into a 60-square foot area. "There is nothing recognizable left," he said. "There must have been an intense fire."

Rhodesian ground and air raids against Nkomo's bases in Zambia in retaliation frr the September plane crash lasted until Christmas and resulted in the death of about 1,000 guerrillas and civilian refugees from Rhodesia.

The attacks were launched while Prime Minister Smith and two of his three black colleagues were on a tour of the United States seeking American suppot. Before leaving, they agreed to attend a British-American sponsored peace conference but only if there were no preconditions.

But Nkomo and Mugabe had already negotiated for a year on an Anglo-American plan calling for a transition to black majority rule under British and United Nations supervision. They thus refused to attend a conference under Smith's conditions. In addition, they were embittered by the raids on their camps in Zambia and Mozambique and increasingly convinced a military victory was in their reach.

In January, the British and American governments concluded it would be impossible to hold a peace conference and warned of the likely ensuing chaos in Rhodesia as the war became steadily worse and spread to neighboring countries.

Since the last Rhodesian raids, Nkomo has moved most of his guerrillas away from the Zambian capital and reportedly divided them into smaller groups to avert heavy casualties.

Zambia has also installed a british air defense system to defend the main international airport outside Lusaka and the capital itself. But it remains doubtful that Zambia would be able to prevent the Rhodesians from carrying out new attacks deep into its territory.