Angolan President Jose Eduardo dos Santos warned today that renewal of U.S. support for the rebel movement led by Jonas Savimbi would pose direct dangers for American economic interests in his country and gravely damage prospects for a regional peace settlement in southern Africa.

Dos Santos, whose Marxist government controls one of the five Third World nations singled out for condemnation by President Reagan at the United Nations today for being "at war with their own people," predicted that Savimbi would use U.S. aid to intensify sabotage campaigns by the National Union for the Total Independence of Angola (UNITA) against American companies and citizens working in Angola.

"If this aid is given to UNITA, the war situation in Angola would be more dangerous," dos Santos said in an interview, adding that such a move would "contradict" and undermine the policies the Reagan administration has said it is following in seeking a Cuban withdrawal from Angola as part of a regional peace settlement in southern Africa.

His remarks provided a counterpoint from the Third World to Reagan's speech, which attached responsibility for instability in Africa and Asia to the presence of Soviet and Cuban troops and advisers, and which described insurgent movements like Savimbi's UNITA as "democratic resistance forces."

The sharply contrasting comments from the two leaders appeared certain to intensify a struggle within the Reagan administration and in Congress over proposals to provide direct backing for Savimbi's forces, which failed to win control of the former Portuguese colony when it was granted independence in 1975. The State Department has said it opposes such aid, and U.S. officials apparently repeated this opposition in talks with dos Santos this week.

Providing aid to Savimbi would be "an act of solidarity by the United States" with the white-minority government of South Africa, which provides extensive aid to UNITA, dos Santos said.

He accused UNITA of wanton killing of civilians and compared the "terrorist" bombing by UNITA of a hotel housing Cuban and Angolan troops in 1983 to the massacre of U.S. marines in Beirut by a suicide bomber the same year.

Dos Santos indicated that attacks by South African forces on his country this year and stepped-up help for UNITA have not only justified but also actually deepened Angola's reliance on the Cuban expeditionary force that has been in the country since l975. He said that the attacks had caused him to reassess the number of Cuban troops Angola would need in the future to protect its territory.

But the soft-spoken Angolan leader, who was trained as a petroleum engineer in the Soviet Union, also disclosed that Angola and the United States this week reopened diplomatic contacts that had been broken off this summer. The State Department confirmed today that Undersecretary of State Michael H. Armacost and Assistant Secretary of State Chester A. Crocker met with dos Santos in New York on Tuesday.

The Reagan administration has sought for four years to mediate an agreement between South Africa and Angola that would lead to a Cuban troop withdrawal from Angola in return for South Africa withdrawing from the disputed territory of Namibia and agreeing to independence for Namibia to be achieved under United Nations auspices.

But the U.S effort appeared to have collapsed after Congress voted this summer to repeal a law banning covert U.S. aid to UNITA. That law was enacted in 1976 following the unsuccessful attempt by UNITA, with backing from South Africa and the Central Intelligence Agency, to oust the government of dos Santos' Popular Movement for the Liberation of Angola.

The U.S. effort already had been set back in May, when a South African commando unit was caught in the northern Angolan province of Cabinda attempting to sabotage a Chevron Oil installation that provides the Angolan government with more than 80 percent of its foreign exchange earnings.

Dos Santos said that the three South African soldiers in the unit were accompanied by six UNITA guerrillas and were carrying propaganda leaflets that would attribute the raid to UNITA.

Speaking in Portuguese, with a translator present, the Angolan president said that despite congressional efforts to provide as much as $27 million in funding for UNITA, his government still wanted to improve relations with the Reagan administration "in terms of dialogue."

He said events of the past year "have very much weakened American credibility as a mediator, but we would not want to say that role has been destroyed."

Dos Santos said he still was prepared to negotiate a phased withdrawal of most of the Cuban troops, a proposal he made a year ago in the context of the American mediation effort. He confirmed that he had then proposed to keep 10,000 to 12,000 Cuban soldiers in Cabinda and around the capital of Luanda for an indefinite period after the phased withdrawal of some 20,000 Cubans from southern Angola, but added that "conditions have altered" since he proposed a residual force of that size.

In his speech today, President Reagan said that there were currently 35,000 Cuban troops in Angola and 1,200 Soviet advisers were supervising combat operations there.

Dos Santos conceded that a major offensive mounted by the Angolan Army last month against UNITA's headquarters at Jamba in southern Angola had been "more or less" stalemated just as it seemed to be on the verge of success.

He said Angolan troops had broken through two defensive lines established by UNITA troops around the town of Mavinga, and were making progress in breaking through the final line of defense when a heavy South African air attack halted them. The Angolan leader asserted that white South African troops had manned the third line of defense around Mavinga.

Dos Santos has emphasized in talks with U.S. officials and businessmen this week that he wants to expand the economic ties that have made Angola the fourth largest trading partner for the United States in sub-Sahara Africa despite the absence of formal diplomatic relations between the two countries. He attended a luncheon today with senior executives of American banks, oil companies and other businesses at Bankers' Trust.

"We have established a difference between commercial relations and diplomatic relations, and we don't intend to alter this in any negative sense," dos Santos said in the hour-long interview in a New York hotel. "We want to enlarge it.

"It is our desire that American businesses use their influence to avoid U.S. aid to UNITA. It is better to do business in a climate of peace and stability, and helping UNITA would undermine that stability. That would threaten the interests of the United States," he said.

"If the South African commando raid had succeeded in Cabinda, American and Angolan citizens would have been killed. The Chevron installation would have been damaged. Is that in the interests of the United States?" he asked.

He noted that UNITA publicly claimed responsibility for an attack that resulted in the death of an American charter pilot earlier this year.

Dos Santos said that Angola is not willing now to return the bodies of the two white South African soldiers killed in May or to hand over the South African major who was captured and who told the details of the operation at a press conference. "We cannot return them unconditionally" as South Africa has demanded, he said, adding, "They were not in Angola on a holiday."

He said the six UNITA guerrillas had escaped during the skirmish in Cabinda.