A Washington lobby group that has previously obtained sensitive classified documents says it has been given a U.S. intelligence report warning that the main guerrilla group operating in South Africa is gaining strength and is weighing a more militant strategy involving attacks on white civilian targets.

A second document made available to The Washington Post by Trans-Africa, a black American lobby group on African affairs, suggests that the white-minority government in South Africa is considering suppression of reliable reports of successful guerrilla attacks to protect white morale.

That report is contained in what Trans-Africa describes as a copy of a page from the April 15, 1982, National Intelligence Daily, a highly classified compilation of current intelligence information from the CIA, the Defense Intelligence Agency, the National Security Agency and other intelligence sources.

It is circulated by the CIA director, as head of U.S. intelligence, to a limited number of senior policy makers in the White House, the Cabinet and elsewhere in Washington and to certain military commanders overseas.

The daily summary said Pretoria was considering new and stricter limitations on publicizing terrorist attacks because "the ANC [African National Conference, the main guerrilla group in South Africa] benefits from press coverage of its attacks." That, in turn, the summary said, "will strengthen the militants in the ANC who want the group to engage in spectacular attacks against whites."

Supporting that assessment is what Trans-Africa Director Randall Robinson said was an extensive CIA report on the outlawed nationalist group. Robinson, who said the report came into his hands in April, refused to make portions of it available for publication. He said they dealt with specific personalities and bases of the ANC.

Separate CIA spokesmen, making what each called the agency's "usual" response to press queries, refused last week to confirm or deny the existence or the reported contents of the excerpt from the intelligence daily as well as the CIA report. In both cases, the spokesmen initially asked for details of the documents and responded several hours later.

Yesterday, a man identifying himself as George Schwegmann, from the CIA's Office of Physical Security, telephoned to inquire about the page of the April 15 intelligence summary. He said he had been advised by the agency's Department of Public Affairs of the copy. He was told that it no longer was in the reporter's possession.

Other sources in a position to be aware of the authenticity of the documents declined to challenge them. In May 1981, Trans-Africa leaked to the press classified State Department documents on Africa policy that U.S. officials later acknowledged to be valid.

The forecast in the alleged CIA report is for "more persistent and widespread racial unrest" in South Africa as the ANC moves against whites' "lives, property and security" to force them to face up to the need for change. The authorities are forseen introducing more repressive measures and mounting further military operations against the group's installations in neighboring states.

The report, a chronicle of the group from its beginnings in the early part of this century through its current and considerable revival after the suppression of the Soweto riots of 1976, echoes widely accepted assessments within academic and business circles here of the ANC's burgeoning strength.

In its review of the ANC's major sabotage operations of the last two years, the report cites "improved efficiency and coordination" of the group's operations between 1980 and 1981, and an increase from 10 to 40 in major attacks against government and economic targets in that period.

"It is clear," the report said, that ANC "could have inflicted a large number of white casualties if had chosen to do so."

Pointing to changes in the group's operations, the report says that "terrorist teams are now beginning to remain in South Africa for longer periods of time, sometimes carrying out several assignments before finally leaving."

It estimates that the ANC has 1,000 to 2,000 active members, meaning those living outside South Africa who have received military training, and perhaps 2,000 to 3,000 others inside South Africa who secretly belong to the banned organization.

The growing power of the militants in the group's leadership, most of whom joined in a major influx after the Soweto riots, "may lead to . . . changes in the group's strategy," the study says, noting that many younger members have objected to what they considered to be the "careful" approach of the more traditional leaders to military operations inside South Africa.

While it predicts "increasing success" for the ANC in filling the vacuum in leadership in South Africa left after the death in custody of black activist Steve Biko, the study judges that the group will pose a "serious threat to white control" in South Africa only if it develops "an effective underground political organization" inside the country.

The ANC has "a long way to go before it can claim a significant political network" inside the country despite polls showing it as the most popular movement among South Africa's 20 million blacks, the study said.