WASHINGTON (AP) _ Chief Mangosuthu Buthelezi, a Zulu leader in South Africa who opposes U.S. sanctions against his white-ruled country, said he told President Reagan on Monday that Washington needs to increase aid to blacks.

''I think it is time for Americans to take stock of the consequences of what they've done,'' Buthelezi said, referring to anti-apartheid sanctions passed by Congress Oct. 2 over the president's veto.

Buthelezi, speaking at the National Press Club after his White House meeting with Reagan and Vice President George Bush, said he told the two men that the sanctions will be ''very damaging to many victims of apartheid.''

''Therefore, it seems to me that they should step up humanitarian aid,'' Buthelezi said, adding that the ''majority of blacks did not want sanctions applied.'' Buthelezi also said he was disturbed to see many big American companies, such as IBM Corp., and General Motors selling their facilities in South Africa.

''American corporations still have a role to play,'' he said, noting that they have often set the pace for improvements for black workers.

The tribal leader, who was on a private visit to the United States, also said he told Reagan that the administration should focus on regional issues in Southern Africa by having top-level U.S. diplomats conduct ''shuttle diplomacy.''

But Buthelezi said he was not convinced that the new U.S. ambassador to South Africa, Edward Perkins, will have much effect on the thinking of white leaders in Pretoria because they are in an angry mood.

Despite Perkins' ''great stature,'' blacks may expect the new black ambassador ''to deliver more than he can,'' Buthelezi said.

''I see Mr. Botha as in a very belligerent mood, in a very defiant mood,'' Buthelezi said, referring to South African President P.W. Botha.

Botha has adopted a ''go to hell'' attitude, particularly in his reaction to sanctions, Buthelezi said.

Last week, Botha criticized a U.S. Agency of International Development program to study malnutrition in the African homelands, calling the plan ''revolting'' and ''insulting.'' An AID official who sought to do the study was denied a visa.

Money for U.S. aid programs designed to provide education and help black South Africans is expected to climb to about $25 million in fiscal 1987, about $5 million more than the previous year.

While Buthelezi favors an increase in U.S. assistance, he said he was unhappy that his Zulu organization, Inkatha, is banned from receiving money because it is labeled a political group. Inkatha claims more than 1 million members.

Buthelezi's positions, particularly on sanctions, have drawn fire from anti-apartheid activists in the United States and in his own country. He acknowledged his ''differences'' with Bishop Desmond Tutu on the sanctions issue, even though he is loyal to Tutu as an Anglican.

Members of the African National Congress, the outlawed black political group, also have attacked Buthelezi's views. But Buthelezi said he has high regard for Nelson Mandela, the ANC leader who has spent more than a quarter of a century in prison.

Buthelezi has said the white Pretoria government's plan for an advisory council of black leaders cannot work unless Mandela is released. The United States also has called on Pretoria to free Mandela.

Referring to American anti-apartheid activists who have criticized him, Buthelezi asked rhetorically, ''What right does anyone have to question my integrity when Mandela doesn't?''

Buthelezi believes apartheid can be ended without violence. Indeed, he said market forces in the economy doom the white government's system of racial separation.

While he ideally favors a system of government based on one-man-one-vote, he said that if violence is to be avoided, blacks must be prepared to negotiate around acceptable comprises, including a federal solution.

Buthelezi is on his second visit to the United States in less than two years. During his trip, he has appeared frequently before public groups and is traveling to Florida, California and Texas.