WASHINGTON (AP) _ President Reagan assured guerrilla chieftain Jonas Savimbi on Thursday he wants to be ''very helpful'' to his campaign to oust the Cuban-backed government in Angola, and sources said the administration has earmarked up to $15 million in secret military aid.

Savimbi, leaving the White House, pronounced himself satisfied.

Reagan, dressed in a dark business suit, and Savimbi, bearded and wearing a Nehru jacket, posed for pictures in the Oval Office, sitting in wing chairs in front of a low-burning blaze in the fireplace.

''We want to be very helpful to what Dr. Savimbi and his people are trying to do, and what we're trying to arrive at is the best way to do that,'' Reagan said.

''We want to be very supportive,'' the president added. ''We're seeking a way to be of help.''

U.S. officials, who declined to be identified, said the administration had notified congressional intelligence committees it would tap a CIA contingency fund to provide assistance for Savimbi, who was trained as a guerrilla fighter by Mao Tse-tung and other leaders of the Chinese revolution before forming the National Union for the Total Independence of Angola (UNITA).

The administration does not need advance approval from the committees to distribute the funds.

The administration has been split over how to support Savimbi, and the issue has surfaced in Congress. Many conservatives favor open assistance and many liberals oppose any aid because Savimbi is aligned with the white-ruled South African government.

Within the next several weeks, the Senate is expected to debate whether to provide additional aid to Savimbi.

House Speaker Thomas P. O'Neill Jr., D-Mass., predicted Thursday that Reagan would have trouble winning House approval for military aid to Angolan rebels.

Given the constraints of the Gramm-Rudman budget-cutting measure, ''any of that money is going to have terrific difficulty,'' the speaker said.

Savimbi's forces control one-third of Angola's territory and exercise political influence over about 60 percent of the country's 7 million people.

On the other side is a Marxist government backed by 35,000 Cuban troops and Soviet aid totaling more than $2 billion in recent years, according to administration estimates.

Until congressional repeal of the so-called Clark amendment last year, the United States had been banned from providing aid in Angola.

Resumption of covert aid would renew - at least partially - the role the CIA played in Angola. It was disclosure of secret CIA assistance to UNITA that led to adoption of the Clark amendment in 1967.

In contrast to the administration, key congressional leaders, including Sen. David F. Durenberger, R-Minn., chairman of the Senate intelligence committee, and Rep. Lee H. Hamilton, D-Ind., chairman of the House intelligence committee, insist that any aid to Savimbi be provided openly, subject to full congressional debate.

However, a senior administration official rejected that approach, saying, ''We don't think it's the right way to go.''

The official, briefing reporters at the White House after Savimbi's departure, said, ''We don't think it (overt aid) will work. We just don't think it's the appropriate way to go in terms of strategy.''

The official, who insisted on anonymity, said the preference for secrecy was based on ''a variety of reasons having to do with diplomacy and practicality. How are you going to get aid in there, for example?''

Presidential spokesman Larry Speakes said the administration will continue pressing its case on Capitol Hill.

''We have committed in the case of Angola to supporting and being helpful ... to Savimbi. We are working with the (congressional) committees the appropriate way to do this,'' Speakes said.

During their private meeting, the official said, Reagan told the rebel leader the United States wants ''to be supportive in ways that are effective.''

Speakes said Reagan was ''impressed with Dr. Savimbi's activities through the years, impressed with his dedication to the goals of democracy and I am sure that impression was enhanced.''

The press spokesman underscored administration arguments that U.S. firms in Angola are helping prolong the war by serving as a source of revenue for the Marxist government.

''Much of the hard currency earned by the Angolan government, with the help of these firms, goes toward military equipment and payment for Cuban troops and their support,'' Speakes said.