A top State Department official yesterday called upon American oil companies operating in Angola to "think about U.S. national interests" in continuing to do business with the Soviet- and Cuban-backed government there.

Assistant Secretary of State Chester A. Crocker, reporting on his latest visit to South Africa and Angola, said the companies should realize they are caught up not only in the Angolan civil war but "a rather hot political debate" in the United States about possible U.S. aid to noncommunist guerrillas fighting the Marxist Angolan government. "They should be thinking about U.S. national interests as well as their own corporate interests as they make their decisions," he said.

Crocker's warning came as the leader of the Angolan noncommunist rebels, Jonas Savimbi, arrived here for a two-week lobbying campaign to gain U.S. military and other aid for his movement. Savimbi was scheduled to meet with Secretary of State George P. Shultz and Secretary of Defense Caspar W. Weinberger today at the start of a round of meetings with administration officials, including President Reagan.

Crocker stopped short of asking American firms to leave Angola. But it was the first time a high-ranking State Department official has suggested that they might be acting against the national interest by helping to finance the Marxist government.

Gulf Oil Co., a subsidiary of Chevron, is Angola's major oil producer and provider of the government's $2 billion in annual oil revenues. Yesterday, the Conservative Caucus, a Virginia-based interest group, held a news conference to discuss its month-old grassroots campaign to force Chevron to quit Angola.

Another U.S. oil company, Conoco, a division of Dupont, is about to sign an agreement with Angola for new off-shore oil exploration and production, according to sources close to the Angolan government. Conoco officials could not be reached.

Chevron spokesman Stephen North indicated surprise at Crocker's remarks. "I can assure you that they have not talked to us in any way, shape or manner about what we're doing there," he said. "We're there on the explicit understanding that it's okay to be there." North said the effect of the Conservative Caucus' campaign against Chevron so far had been "zero" and that the company had received "less than 3,000" protest cards of 70,000 the group had sent to its supporters to mail to the oil company.

Crocker said he had "no breakthroughs" to report on his latest talks with Angolan and South African officials regarding a negotiated withdrawal of Cuban troops from Angola and independence for neighboring South African-administered Namibia.

"Confidence between the parties remains low," he reported. But both the South African and Angolan governments asked him to continue his mediating role, and he said he had relayed "some ideas" from the Angolans to the South Africans on ways to break the impasse.

Crocker indicated there was new flexibilty on both sides that permits continuation of the U.S. search for a negotiated settlement. South Africa took a "significant step" in November by agreeing "in principle" to a U.S.-proposed timetable on Cuban withdrawal from South Africa and a South African withdrawal from Namibia.

The Angolans, in turn, had indicated that "under certain conditions" they were prepared to move "substantially" beyond their present position, Crocker said, although he declined to elaborate.