Angola's Marxist government has launched a surprise rainy-season offensive involving Cuban troops trying to capture rebel headquarters in southern Angola, rebel spokesmen said here yesterday.

U.S. officials said they could not confirm the report, broadcast by South Africa's state-run radio Tuesday, and expressed doubt that the government would begin a full-scale offensive at this time because of weather conditions.

"We're not convinced an offensive is under way," a Defense Department source said. "We think it's too early to tell."

The report's accuracy is important because a new government offensive could trigger a U.S. decision to speed covert lethal and nonlethal aid that the Reagan administration has decided to provide to the rebel group, the National Union for the Total Independence of Angola (UNITA).

The administration has tentatively approved initial aid of between $8 million and $15 million to be implemented by the Central Intelligence Agency, according to congressional and administration sources.

UNITA and South Africa have been urging the United States to become more directly involved on the guerrillas' side in the Angolan civil war, which has been fought for a decade.

The administration has reportedly decided, however, to await the outcome of U.S. diplomatic efforts to revive negotiations between South Africa and Angola toward a southern Africa settlement before dispensing any aid.

The U.S. mediation plan aims at getting Angolan officials to agree to withdrawal of the 35,000 Cuban troops from Angola and at persuading South African officials to allow U.N.-supervised elections for the independence of Namibia.

Assistant Secretary of State Chester A. Crocker spoke in Lusaka with Angolan government officials Nov. 27-28 and later reported that South Africa and Angola showed interest in renewing the negotiations.

He is expected to meet with the Angolans again soon.

Meanwhile, Reuter news agency reported yesterday that South Africa sent a small contingent of soldiers into Angola in search of nationalist guerrillas fighting for the independence of neighboring South African-ruled Namibia.

In September, South Africa also sent a force into Angola, but its troops wound up helping UNITA stave off another Cuban-backed government offensive.

South Africa's state-run radio said Tuesday that thousands of Angolan troops, aided by Cubans, were preparing a new assault on UNITA headquarters at Jamba in southern Angola.

UNITA spokesman Figueiredo Paulo said the new government offensive began Dec. 6 on three fronts and involved a Cuban company of troops from the 54th and 71st brigades.

The UNITA representative here said one objective of the Angolan drive was to spread out UNITA forces and prevent their concentration for defending Jamba.

Paulo said an immediate objective of the offensive was to capture the town of Cangamba, which is 200 northeast of Jamba and where there is an airport.

"If they can reach Cangamba, they can attack Jamba by air ," he said.

U.S. officials have been evaluating UNITA reports of a new Angolan offensive for at least a week but have been unable to confirm that the renewed fighting is anything more than tactical skirmishing.

"We are looking at the evidence very carefully," one U.S. official said. "They may be trying to keep UNITA off balance."

The Defense Department source said a new offensive against UNITA had not been expected until February or March and that present movement on the ground might be only "tactical maneuvering" in preparation for a later push.

The official suggested that the fighting might reflect new confidence by Angola's ruling Popular Movement for the Liberation of Angola (MPLA).

The MPLA has just completed a congress reaffirming the present leadership's position, the continuing close relations with the Soviet Union and Cuba and a tough line toward UNITA.