In a last-ditch effort to avoid being locked out of negotiations among the United States, South Africa and Angola aimed at a regional peace settlement, Angolan rebel leader Jonas Savimbi has threatened to unleash a major guerrilla offensive against the capital of Luanda next month unless he is included in the talks.

Speaking to correspondents at his headquarters settlement of Jamba, in the remote bushlands of southeastern Angola, Savimbi said yesterday that his military commanders would leave today to start preparing for the new offensive against the capital, nearly 800 miles to the northwest.

Threatening once again to carry his nine-year guerrilla war into Luanda, Savimbi hopes to force President Jose Eduardo dos Santos to agree to his being given a place at the negotiating table and for this to lead to his inclusion in a government of national unity that would rule for an interim period until elections can be held. Dos Santos opposed this categorically in an interview last month with The Washington Post.

Dos Santos said that Savimbi had "betrayed Angola and betrayed Africa" by cooperating with segregationist South Africa and that he and his collaborators would face certain death if they attempted to return to Luanda.

The ruling Popular Movement for the Liberation of Angola (known by its Portuguese initials MPLA) joined with Savimbi's National Union for the Total Independence of Angola (UNITA) and another rebel group in a brief interim government leading up to independence in November 1975, but the accord degenerated into civil war. Diplomatic observers believe this is a major factor in dos Santos' adamant stance against Savimbi's being included in such a government again.

In an interview yesterday, Savimbi dismissed this as political posturing, saying that under enough pressure he was sure the president would negotiate with him. He added: "In any case, if I ever get to Luanda, I will be more popular there than he is."

The tripartite negotiations reportedly have reached "an extremely delicate phase," according to a senior U.S. official, and a deal could be coming together, with the Angolans agreeing to a phased withdrawal of most of the estimated 25,000 Cuban troops in their country in return for South Africa giving neighboring Namibia its independence and stopping all aid to Savimbi.

If an agreement results from the current negotiations, Savimbi would be sandwiched between the Marxist-led Angolan government to the north of him and an independent Namibia, probably ruled by the equally hostile, black nationalist South-West Africa People's Organization, or SWAPO, to his south.

He would be cut off from vital supplies, particularly fuel, from white-ruled South Africa, which now controls the former German colony of Namibia in defiance of the United Nations.

Savimbi seems to hope that by threatening to step up the war he may persuade South African President Pieter W. Botha either to make the inclusion of UNITA in the negotiations a new precondition for settling the Namibian issue, or at least to stall long enough to give his strategy time to work.

It is an uncertain stratagem, for Savimbi knows he does not have the military capacity to overthrow the distant Luanda government, but only to harass it.

Savimbi, who said he would add 7,000 crack troops to those already in the area of the capital, previously has threatened offensives against Luanda. Last month, rebels destroyed several power transmission towers and high-tension lines 100 miles southeast of Luanda, shutting off power in the capital for several days. Savimbi's forces also claim to have damaged two ships in the capital's harbor recently.

[Diplomatic sources in Luanda estimate that Savimbi has several hundred to 1,000 men operating within a 100-mile radius of Luanda.]

Savimbi made it clear yesterday in interviews here that he feels he can count on strong sympathy for his position from the South Africans, but, as he told the correspondents yesterday: "Pieter Botha is my friend, but I know that he has to look after the interests of his country first."

His strategy appears, therefore, aimed at securing a place at the negotiating table before any deal is concluded that would make it logistically difficult for South Africa to continue supplying his forces.

He told the correspondents at a press conference: "UNITA will have to be party to all negotiations whose objective will be to determine the future of Namibia and the corresponding withdrawal of Cubans from our country.

"We insist that all the parties involved in the southern African conflict should recognize UNITA as a central factor with sufficient means to influence the [Cuban] withdrawal.

"I don't want to rock the boat, I want to be part of it," Savimbi added.

The United States and South Africa have insisted on withdrawal of the Cubans from Angola as a precondition for implementing a U.N.-approved independence formula for Namibia.

For more than two years Angola, backed by other African states in the region, has refused to accept this "linkage" of the Cuban issue, claiming it is a violation of its sovereign rights, but there have been signs lately that dos Santos is becoming increasingly receptive to an end to the prolonged conflict, which has brought his country to the edge of ruin.

The first significant step toward a settlement came in February when Washington helped arrange a disengagement agreement between South Africa and Angola under which South Africa would withdraw an invasion force that had penetrated 120 miles into Angola during fighting last year and stayed there.

In return, Angola promised that the vacated territory would be kept clear of SWAPO guerrillas trying to cross into Namibia, where they are fighting a war of independence, and a joint South African-Angolan monitoring commission was set up to police the arrangement.

Although South Africa has halted its withdrawal 25 miles north of the border, apparently to maintain a negotiating position, this agreement increased American credibility with the Angolans and encouraged dos Santos and a group of pragmatists grouped around him in the Marxist government to work more closely with the U.S. mediators.

A new move forward came Oct. 18, when Frank Wisner, the deputy assistant secretary of state for Africa who has been a frequent visitor to Luanda during the past two years, returned to Washington with an Angolan offer that held promise of a breakthrough on the Cuban stalemate.

A senior State Department official said at the time that once South African troops have withdrawn from Angola, removal of the Cubans will be "a part of the package" of overall peace in the region.

A week later dos Santos dismissed his foreign minister, Paulo Jorge, who is perceived by some western diplomats as closer to Moscow than other officials in the Luanda government and who has been skeptical of U.S. diplomatic efforts in the area and a key figure in resisting any yielding on "linkage."

The United States conveyed the Angolan offer to South Africa at a meeting on the Cape Verde Islands 10 days ago, and South Africa is expected to respond by Thursday.

The view in Pretoria is that it is an important acceptance by Angola that a removal of the Cubans is now a negotiable issue, and South Africa is expected to respond with a counterproposal.

This turn of events apparently alarmed Savimbi, who hurriedly called a "special congress" of 400 senior UNITA commanders to discuss the implications of the new negotiations, and invited South African and foreign correspondents to attend the final parade-ground ceremony of this congress in Jamba yesterday.

To reach Jamba involves a four-hour flight from Pretoria across the Kalahari Desert in a 40-year-old chartered Dakota, followed by a bone-crushing, four-hour truck drive along bumpy sand tracks that wind through the thick southern Angolan bush.

The remote headquarters settlement, about 100 miles north of the Namibian border and 150 miles west of the Zambian border, is testimony to Savimbi's drive and efficiency. It is a makeshift capital of reed huts, built in an undeveloped part of the country that the Portuguese colonists used to call "the end of the world" and kept as a hunting reserve, to which the enterprising Savimbi has brought all the modern conveniences of a well-run safari camp.

Street lamps looted from raided towns light the rudimentary roadways. There is a banquet hall cooled by electric fans, and the correspondents dined on venison stew and South African wines served with good linen, crockery and glassware.

The parade-ground ceremony featured some precision drilling by about 2,000 well-trained troops and a parade of equipment captured in the recent campaigns, including trucks, artillery pieces, armored cars and two Soviet T34 tanks.

Addressing the crowd on the new negotiations, Savimbi said they were part of the Luanda government's attempt to switch from a failed military campaign to a bid for a diplomatic solution in its campaign against UNITA.

The government was "falsely . . . giving the impression that the departure of the Cubans is acceptable, while proposing the timetable of their departure in such a manner as to facilitate the implementation of U.N. Resolution 435 [the Namibian independence formula], thereby benefiting SWAPO and gaining access to UNITA's logistical routes," the insurgent leader said.

Enlarging on this at a press conference later, Savimbi said he had reliable information that the Angolan government planned to give citizenship to some of the Cubans, and had also landed 2,500 black Cubans in the country within the past three weeks.

"Who is going to tell the difference between black Cubans and black Angolans?" he asked, saying this was a clear indication of the Luanda government's intended duplicity.

Savimbi said UNITA was not prepared to accept being "traded in for a fictitious withdrawal of the Cubans."

The rebel leader implied a certain feeling of bitterness toward the U.S. State Department when he said there had been "jubilation" in Jamba at the reelection of President Reagan, but he felt it was time for the State Department to "unequivocally clarify" its policy on southern Africa.