Two of the three U.S. intelligence agencies have concluded that the pro-Western guerrilla movement led by Jonas Savimbi has no chance of winning on the battlefield against the Marxist Angolan government and little prospect of joining a coalition, according to congressional and intelligence sources.

Nevertheless, the sole agency that predicts a possible military and political victory by Savimbi -- the Pentagon's Defense Intelligence Agency -- has for several months shared intelligence information with him and provided communications support to his guerrillas, according to Pentagon and other sources.

One source said the intelligence-sharing and other nonlethal aid, which reportedly began in December, was the first part of a two-stage covert assistance program approved by President Reagan and his national security advisers last November. Savimbi, who has been lobbying for U.S. military aid here since early last week, has met with Secretary of Defense Caspar W. Weinberger and held three sessions -- two of them hastily squeezed into his hectic lobbying schedule this week -- with DIA officials to receive fresh intelligence reports and discuss his specific military needs, these sources say.

Contrary to the DIA's optimistic assessment of Savimbi's prospects, the Central Intelligence Agency and the State Department Bureau of Intelligence and Research have concluded that he "can't win and can't force a coalition government" on the Marxist leaders in Luanda, according to two sources familiar with the evaluations of all three intelligence agencies.

The DIA holds the view that Savimbi could prevail if he holds on long enough to provoke dissension within the Luanda government and triggers a military coup that would favor his cause, one source said.

The conflicting assessments of Savimbi's prospects by analysts of the CIA, DIA and Bureau of Intelligence have come in the midst of an intense debate within the administration and Congress over whether the United States should resume its involvement, after a decade of abstinence, in the Angolan civil war.

One intelligence source said the three agencies were not only divided among themselves in their assessments but in some cases "the lower level was not agreeing with the top" within individual agencies. Some judgments at the top on these issues appeared to reflect "political wishful thinking" rather than "hard-nosed, on-the-ground assessments," the source said.

CIA Director William J. Casey was previously reported among those in the administration most supportive of proposals to provide Savimbi with assistance in his struggle. Savimbi lunched with Casey last week and also met with CIA officials who are involved in planning U.S. covert support for his guerrilla forces, according to one informed source.

CIA analysts, however, believe the proposed level of U.S. military aid for Savimbi's struggle is insufficient to make a difference in the military equation on the ground or force any change in the Marxist government's refusal to discuss a coalition with the rebels.

The Reagan administration has approved an initial $10 million to $15 million covert military aid program, but congressional conservatives want a much larger U.S. commitment of either covert or overt aid. A major administration objective in helping Savimbi is to urge the Angolan government to accept reconciliation with Savimbi and give his National Union for the Total Independence of Angola (UNITA) a place in the government.

Savimbi was one of two pro-Western leaders aided by the United States at the start of the Angolan civil war in 1975, when the CIA spent $32 million trying to prevent the Marxist faction from seizing power. However, in January 1976, Congress passed legislation ending CIA involvement in the war.

As in the mid-1970s, the administration, Congress and the intelligence community are again deeply divided over the wisdom of entanglement in the Angolan conflict.

Conservatives have cast the Savimbi aid issue in terms of an ultimate test of the so-called "Reagan Doctrine," the administration's declared objective of aiding anticommunist "freedom fighters" in their struggles against Soviet- and Cuban-backed governments.

"The Reagan Doctrine pretty much hinges on Angola," remarked a House conservative staffer involved in the campaign to help Savimbi.

"It will also set a precedent for the contras," he added, referring to the administration's recently announced plan to ask Congress for up to $100 million in military and other aid for the U.S.-backed anticommunist forces fighting the Sandinista government in Nicaragua.

Opponents of U.S. involvement in Angola, who include 500 academic specialists on Africa who have petitioned Congress to block aid to Savimbi, are warning that the United States is heading for an entangling, and inevitably damaging, alliance with South Africa that will have repercussions for American foreign policy in black Africa.

The debate within the intelligence community has centered on a number of interrelated issues, including Soviet intentions in Angola, Savimbi's military and political prospects, and his strategy in seeking to battle the military pressure on his forces.

Savimbi and his conservative backers here have made much of the growing Soviet and Cuban involvement in the war on behalf of the Angolan government. The pro-UNITA voices point to the arrival of $2 billion in Soviet arms, the increase in the number of Cuban troops to 35,000 and the peril of annihilation that UNITA forces face from an expected large government offensive this spring.

CIA Director Casey reportedly believes Soviet leader Mikhail Gorbachev was personally responsible for upping the Soviet-bloc ante in Angola, according to intelligence sources. U.S. analysts say, however, that the Soviet arms buildup and increase in Cuban troops began in early 1984 and was mostly completed by the time Gorbachev came to power last March.

Savimbi and his backers here also argue that the Soviets and Angola's Marxist rulers are responsible for the recent escalation. But U.S. intelligence sources say there is strong evidence that the Angolan government has been responding to an initial UNITA escalation that began in 1983. That UNITA escalation was supported by considerable South African military and logistical aid, and was launched with the publicly declared intent of massing 20,000 guerrillas in Luanda Province by the end of 1984 for a major attack on the capital.

Savimbi made such good progress in spreading his guerrilla war from his stronghold in southeast Angola to the northeast and northern parts of the country that South Africa's military intelligence had concluded that his forces stood a good chance of achieving a military victory, according to one U.S. intelligence source.

One well-informed source said that the recent DIA support for Savimbi has included upgraded communications equipment as part of the nonlethal phase of covert assistance. Details of that equipment were unavailable, although Savimbi's geographically dispersed forces are believed to need radio equipment to link them together.

The DIA, which reportedly has ties to South African military intelligence, adopted this view, the source said. The Pentagon agency still holds that UNITA has the potential, if sufficiently aided by the United States and others, to fight government forces to a stalemate.

The argument being made by some DIA and other intelligence analysts is that there is "a very strong likelihood" of such a stalemate and a "fairly strong" prospect for a coalition government including UNITA, the source added.

Other CIA analysts have concluded, however, that UNITA does not have the military strength to create such a stalemate, particularly with the Soviets and Cubans increasingly committed to the Marxist central Angolan government. They believe Savimbi is being drawn into a conventional-style war of defending fixed positions, especially his headquarters in Jamba, and that such battle tactics may be catastrophic to his forces.

These analysts say Savimbi should give up this strategy and return to a more mobile style of warfare in which U.S. antiaircraft missiles and antitank guns -- the main two items he is seeking from the United States -- would not be so crucial to UNITA's survival.