A top Reagan administration official said yesterday "the decision has been made" and the "process is in motion" to provide Angolan rebel leader Jonas Savimbi with military assistance in his struggle against the Marxist Angolan government.

Appearing before the Senate Foreign Relations Committee, Assistant Secretary of State Chester A. Crocker asked for "a clear statement from Congress" supporting a continuing U.S. commitment to "negotiated resolutions in southern Africa" but also warning that the United States will not tolerate other countries taking advantage of this policy "to pursue their own aggressive ends."

At the same time, he indicated the administration does not want legislation mandating military assistance to Savimbi's movement, saying that this should be a topic for cooperation and consultation between the executive and legislative branches.

Crocker's disclosure that the U.S. covert aid program to Savimbi has begun came when Sen. Christopher J. Dodd (D-Conn.) asked why yesterday's hearing had been called. Noting that the hearing notice called for a discussion of policy options in Angola, Dodd asked whether the administration had not already reached a decision on covert aid.

Crocker replied, "The decision has been made and the process is in motion. I can't go beyond that."

The hearing failed to resolve the issue of whether the Senate, by a resolution, should give the administration the political approval it had earlier sought for its planned covert supply of $10 million to $15 million in military aid to Savimbi.

During the Angolan guerrilla leader's recent visit in Washington, a disagreement between the Senate Republican leadership and Republican conservatives over the precise wording of a resolution of support resulted in no action being taken before his departure.

Conservatives, leery of the administration's commitment to provide military aid immediately to Savimbi, have been seeking a resolution specifying the material hardware, namely antitank guns and antiaircraft missiles, that the rebel leader has requested to help him fight Soviet tanks and helicopter gunships in an expected spring offensive.

The administration, on the other hand, wanted only a general resolution of moral and material support, leaving the details of a military aid program to the executive branch.

Following Crocker's testimony, Sen. Richard G. Lugar (R-Ind.), committee chairman, said it was unclear whether there would be a markup on any of the three pending bills or resolutions calling for aid to Savimbi.

A Republican Senate aide said the need for any resolution may have been "overtaken by events. I think it's a dead issue."

Sen. Jesse Helms (R-N.C.) repeatedly pressed Crocker to state when military shipments to Savimbi would begin, telling him that "you better get cracking if you're genuine in wanting to aid Savimbi."

Crocker said the aid would be "relevant, effective and appropriate and that means as soon as we can."