Zaire has asked the United States "for assistance in the form of materiel" to help repel what it calls an invasion by "foreign mercenaries" across its border with Angola, the State Department said yesterday.

Other sources said that Zaire has asked the United States for "a broad spectrum" of military equipment to cope with the extremely murky conflict under way since March 8 in the former province of Katanga, now known as Shaba.

Carter administration officials were unusually secretive about what Zaire is requesting, and what they intend to do about it. This sensitivity is partially a product of the tortuous history of the region.

Official sources maintained that the dimensions of the conflict, the forces involved, and their objective, are all unclear, so far as the United States is concerned. But several sources said that what appears to be happening is a retaliatory action arising from the Angolan civil war, in which the United States and Zaire supported the losing side.

The two losing factions in Angola's civil war in 1975-76 were the National Front for the Liberation of Angola, headed by Holden Roberto, and the National Union for the Total Liberation of Angola, led by Jonas Savimbi. Both were supplied with Central Intelligence Agency funds, until Congress cut off all further support in late 1975.

Agostinho Neto's Popular Movement for the Liberation of Angola (MPLA) won that war, with massive materiel support from the Soviet Union and from 12,000 to 15,000 Cuban troops, most of which remain in Angola. Fighting on the side of the MPLA also were several thousand Katanga gendarmes, remnants of the forces led by the late Moise Tshombe's ill-fated independence movement in Katanga in the 1960s, when Zaire was known as the Congo, a former Belgian colony.

Officially, that conflict is over in Angola. But guerrilla warfare continues, with Savimbi's troops and Roberto's both still engaged, although the United States and Zaire have disclaimed any further involvement with either force.

Last Thursday, Zaire announced that "mercenaries" from Angola had occupied three important mining and communications centers in the province formerly known as Katanga.

The "mercenaries," U.S. sources believe, are largely the former Katanga gendarmes, now turned loose against the American-supported government of Zaire President Mobutu Sese Seko, who crushed the Katangese separatists in the 1960s.

American specialists suspect that this represents Angola's retaliation against Zaire, which it charges is still supporting Esberto's National Front, which Zaire denies.

Angola denies any involvement in the incursions across the Zaire border, while in Paris a group calling itself the Congress National Liberation Front has described the attacks as a national uprising by the Katangese separatist movement.

None of this complex background was discussed yesterday by official U.S. spokesmen. State Department spokesman Frederick Z. Brown said, "The Zaire government has asked for assistance in the form of materiel. We are currently considering this request. I am not in a position to give you any kind of details on the request, however."

One source said the Carter administration is considering only non-military aid to Zaire, such as food, clothing medicine.

Zaire, rich in copper and other resources, but with a battered economy, this year is receiving $30.2 million in U.S. military sales credits, with the Carter administration seeking an increase to $30 million in military sales credits, and $2.54 million in military training, for the coming fiscal year.

Sen. Dick Clark (D-Iowa), who led the drive to cut off CIA aid at $30 million in the Angola civil war, said yesterday, "I think we ought to be very cautious about getting involved in a kind of civil war situation" in Zaire, or between Zaire and Angola.