The Reagan administration has tentatively decided to provide less than $30 million as the first installment of a covert aid program to rebels fighting the Marxist regime in Angola, according to administration sources.

However, disbursement of the aid has been delayed until at least January because the State Department hopes to use the threat of U.S. backing for the rebels to pressure the Angolan government into negotiating the withdrawal of 35,000 Cuban troops in Angola as part of a peaceful settlement in southern Africa, the sources said.

Although a White House official indicated that no formal authorization of covert aid has been issued, other administration sources said an informal decision was reached at an interagency meeting in mid-November. The program would initially involve nonlethal aid followed by military assistance for the National Union for the Total Independence of Angola (UNITA) led by Jonas Savimbi, according to one source.

This source said the Central Intelligence Agency had proposed providing an initial aid program of roughly $15 million, half the $30 million figure circulating in administration circles, and a small fraction of the $200 million to $300 million suggested by some officials.

Secretary of State George P. Shultz, according to congressional sources, has effectively lobbied to kill several congressional proposals for $27 million in either humanitarian assistance or overt military aid. Within the administration, Shultz has also prevented the last step in the interagency process leading to a formal presidential finding in favor of covert aid.

Shultz has used the threat of U.S. aid to UNITA to pressure the Angolan government into further concessions on a withdrawal timetable for the 35,000 Cuban troops stationed in Angola. That issue is a key to U.S. strategy for breaking the impasse in negotiations for the independence of neighboring South African-administered Namibia.

On Nov. 27-28, Assistant Secretary of State Chester A. Crocker met for the first time in five months with Angolan negotiators in Lusaka, Zambia. More U.S.-Angolan talks are planned, probably later this month.

Shultz, according to congressional sources, has skillfully used the flurry of diplomatic activity to bolster his argument against the immediate start-up of covert aid.

On Friday, the secretary told a news conference that there had been "some responsiveness" from both South Africa and Angola to a U.S. proposal made last spring on the issues of Cuban withdrawal and Namibian independence. But "that doesn't mean that we're somewhere near a settlement," Shultz warned.

Nevertheless, even the vague hope of progress apparently has been sufficient to fend off Defense Department and CIA pressure to begin immediate covert assistance, according to sources familiar with the debate.

As with covert aid to the contra rebels fighting the leftist regime in Nicaragua, the proposed aid program to UNITA has been remarkably open for an ostensibly secret undertaking. In a meeting with journalists on Nov. 23, President Reagan said, "We all believe that a covert operation would be more useful to us and have more chance of success right now than the overt proposal that has been made in the Congress."

Shultz has finessed his own stand on this issue in recent public statements by saying he supports "those who fight for freedom" but opposes bills in Congress for overt aid to UNITA because "our desire is to support them effectively," as he phrased it last Friday.

The best explanation of what Shultz believes to be "effective" aid right now came during an NBC television interview Nov. 24.

"We believe that if there can be a political solution, a negotiated solution, to the problems of Angola as well as the problem of Namibia, linked as it is to the difficulties in southern Africa generally and in South Africa, that's the way to go and we're trying to do that," he said.

Shultz is believed to seek both South African and Angolan support for a de-escalation of the fighting in Angola, thereby lowering the likelihood of U.S. involvement. South Africa's new deputy foreign minister, Ron Miller, predicted here last Thursday that the Angolan government would launch a second major offensive against UNITA in the next two or three weeks.

Efforts by UNITA supporters in Congress to pass legislation this year providing either overt humanitarian or military aid to the Angolan rebels appear to be running out of steam.

Last Tuesday, the House Rules Committee voted against allowing any amendments, including one calling for $27 million in humanitarian aid to UNITA, to the omnibus government appropriations bill.