President Reagan welcomed anticommunist Angolan leader Jonas Savimbi to the White House yesterday and pledged that the United States would be "very helpful" to his fight against the Soviet-backed Marxist Angolan government.

"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 as he posed for photographs with the Angolan leader in the Oval Office.

The president did not specify what kind of aid the administration intended to provide to Savimbi, who arrived here Tuesday on a 10-day lobbying campaign to win U.S. diplomatic and military backing for his decade-long struggle.

Savimbi said he was satisfied by his 15-minute session with Reagan but declined to specify whether he had been promised any aid, and whether it would be overt or covert. "It is the president who knows what he is going to do," he said.

On Wednesday, Savimbi met with Secretary of State George P. Shultz for more than an hour. This was followed by a half-hour session with Defense Secretary Caspar W. Weinberger to discuss his military needs. Savimbi aides have said what they require most is antitank weapons and antiaircraft missiles.

Pentagon sources said Weinberger and his aides had discussed the possibility of providing Savimbi with nonlethal aid as an interim step under an amendment to the Defense Department authorization bill this fiscal year that allows it to provide excess supplies and equipment for relief purposes "around the world." The measure was intended initially as a method to provide aid to Afghan refugees but has been interpreted by the Defense Department to allow shipments to other anticommunist guerrilla movements, according to the Pentagon sources.

Yesterday, Rep. Bill McCollum (R-Fla.), author of the amendment, and Sen. Orrin G. Hatch (R-Utah), a supporter of aid to Savimbi, sent a letter to Reagan urging him to use the measure to send immediately "substantial amounts" of excess Defense Department supplies, such as uniforms, boots, vehicles and medicine, to Savimbi's forces.

Administration officials have indicated they want to provide Savimbi with covert military assistance and other aid, and oppose a drive by conservatives to get congressional approval for open aid.

A senior administration official said Reagan and Savimbi had held "a very positive exchange" in which the Angolan leader had made known "what his desires, what his goals are." Chief among these, the official said Savimbi told Reagan, was reconciliation between his movement and the Marxist Angolan government.

Reagan told Savimbi the United States had undertaken "a negotiated approach" to achieve this goal, including talks with the Soviet Union about "the Angolan situation." Reagan "will probably continue" to raise the issue at future superpower summit meetings, according to the official.

Reagan's warm welcome and support for Savimbi yesterday set the stage for the drive by conservative groups and other Savimbi backers to obtain U.S. assistance before an expected Angolan government offensive in the spring.

The official said the administration opposes congressional "legislation mandating aid" because of "diplomacy" and "practicality." Asked to elaborate, the official replied, "Well, how are you going to get aid in there, for example."

It is not "the desire" of anyone in the administration to send U.S. aid to Savimbi through South Africa, which has supported the Angolan rebels for years, the official added.

This was the first hint that the administration may be planning to channel a proposed $10 million to $15 million in covert aid through Zaire, or another black African country, to avoid charges that the United States is colluding with South Africa by providing assistance to Savimbi. South Africa is presently his main outside backer and source of arms and supplies.

Although some groups opposing U.S. aid to Savimbi have suggested that covert aid has already begun flowing, a White House official said there was "still no official decision" and that the issue remains "a matter of discussion within the administration and with Congress."

In a related development, the Senate Foreign Relations Committee held a closed-door session yesterday to hear CIA and State Department evaluations of the Angolan war. Administration opposition to public aid for Savimbi has left his congressional supporters uncertain of what course to follow.

Administration officials have said they would welcome a congressional resolution of support for Savimbi. Senate Majority Leader Robert J. Dole (R-Kan.) in December introduced such a resolution but also proposed "material assistance" to Savimbi and economic sanctions against Angola.

Congressional sources said Dole was considering reintroducing the resolution but was still working with the administration and Savimbi supporters on the wording.