The Angolan rebel movement UNITA appears to be overreaching its military capacity and is showing signs of internal dissent as it intensifies guerrilla and diplomatic activity aimed at securing a significant role in negotiations toward a regional peace settlement, diplomatic observers say.

Rebel leader Jonas Savimbi's determination to achieve impressive military successes to demonstrate the insurgents' strength has met with opposition from field commanders who fear that he is pushing the guerrilla offensive ahead too far and too fast, according to the assessment of western analysts here.

Recent tensions within the pro-western rebel movement have been reflected in the rebels' acknowledgment of a setback in plans to advance toward the capital of Luanda, open disenchantment over closer U.S. ties with the Marxist government and uncertainty about the status of top guerrilla leaders.

"Something is clearly happening inside the UNITA hierarchy," said a western diplomat in Lisbon, "but it is difficult to assess the precise significance of the changes." Most Africa-watchers here attribute the signs of agitation to rebel fears that they are being squeezed out of U.S.-led diplomatic efforts to end bush wars in Angola and the neighboring South African-controlled territory of Namibia.

UNITA, a Portuguese acronym for the National Union for the Total Independence of Angola, has been waging an effective antigovernment campaign of economic sabotage and guerrilla attacks since its defeat by the ruling Popular Movement for the Liberation of Angola (MPLA) in a brief civil war that followed independence from Portugal in 1975. The Marxist MPLA draws on extensive Soviet technical aid and the backing of about 25,000 Cuban troops in its war against the rebels, who acknowledge logistical support from South Africa.

Successive U.S. administrations have given UNITA tacit support as allies in a strategy to diminish Soviet and Cuban influence in southern Africa, although Congress banned U.S. aid to the rebels in 1976. But the decision by President Jose Eduardo dos Santos last fall to respond to three years of quiet U.S. diplomacy in the region has led to a warmer climate in relations between Luanda and Washington.

"The U.S. seems to be forgetting that we are its real friends in Angola," said UNITA information officer Fernando Wilson dos Santos. "Washington is acting against its own global interests by asking us to make concessions and not the MPLA." He expressed concern that the Reagan administration would seek to accommodate Luanda by accepting a settlement in which UNITA had only a symbolic presence in a future Angolan government.

While western observers recognize that the rebels' striking power is considerable, they note recent indications that their forces may have been strained by a widening of the offensive. During the past year UNITA claims to have extended operations into every Angolan province except southwestern Mocamedes, where the desert terrain is unsuited to UNITA's style of combat. The rebels say they control large areas of the southeast and are setting up their own administration in the remote northeastern diamond-mining province of Lunda.

But rebel officials admit that they have been stalled in their declared aim of massing 20,000 guerrillas into Luanda Province by the end of last year in a drive toward the capital. "So far only about 7,000 men are in the province, but the mobilization is continuing," said Wilson dos Santos. That number is thought to be exaggerated. Savimbi claims to command a force of 50,000, including regular troops, guerrillas and local militias, but reliable independent sources estimate rebel strength at about 20,000.

At the same time, the Angolan government has reported a succession of victories. "There has been a crescendo of claims of military successes over the past year," said a western diplomat in Luanda contacted by telephone. "But it is difficult to judge if this reflects reality or is simply a government attempt to match UNITA's propaganda output." The diplomat said that the capital was enjoying its longest period of calm in several years.

Government claims of military successes have coincided with a spate of unconfirmed reports alleging defections and demotions from the UNITA leadership. Savimbi appeared in a news clip on South African government television Jan. 12 to deny that he had been killed, captured or wounded in a government raid on his bush headquarters in southeastern Angola. Diplomats said rumors to that effect had circulated in Luanda for three weeks before Savimbi's television appearance.

The UNITA hierarchy has undergone a reshuffle aimed at greater efficiency, Wilson dos Santos said, with UNITA chief of staff Demontenes Amos Chilingutila rising to the post of operational commander.

The Angolan goverment claimed that Chilingutila had been expelled from the movement in disgrace following a series of defeats. Similar reports said another UNITA commader had defected with his troops to the South-West Africa People's Organization (SWAPO), the nationalist guerrilla movement fighting for Namibian independence.

Western observers here said the government reports were probably the result of attempts by Angolan military intelligence to aggravate differences within UNITA. They believe the focus for discord is opposition from commanders in the field to ambitious military aims favored by Savimbi.

A likely reason for the shake-up, observers say, is to get UNITA members who have been enjoying a relatively high standard of living ouside Angola back to the bush to keep them sharp and avoid antagonism within UNITA ranks.

Wilson dos Santos said he would be returning to the field this month to assume his position as UNITA head of information. He has been acting as the organization's spokesman in Europe.

U.S. officials have made it clear that they consider UNITA to be an internal Angolan problem and not part of the equation in seeking a southern African peace settlement. The main issue for the Reagan administration is for Angola and South Africa to reach agreement on a Cuban pullout from Angola tied to a withdrawal of South African troops from Namibia and the implementation of a United Nations independence plan.

Western diplomatic sources said U.S. Assistant Secretary of State for African Affairs Chester A. Crocker held private talks with Savimbi in Cape Town during a recent visit. The U.S. Embassy issued a statement saying the United States regularly maintains contacts with UNITA but does not always specify times and places. The embassy declined to confirm or deny the reports of Crocker's meeting with Savimbi.

South Africa issued a statement later saying Foreign Minister Roelof F. (Pik) Botha held talks with Savimbi but did not elaborate.

Angola, which has hardened its insistence that South Africa cut off all aid to rebels, would like to see more U.S. pressure on Pretoria to this end. UNITA acknowledges that losing South African support would be expensive but claims it could survive on airlifts of arms from unspecified nations to landing strips in the Angolan bush.