CAPE TOWN, SOUTH AFRICA, JAN. 31 -- Angola has accused South Africa of resuming military support of U.S.-backed rebels fighting in southern Angola just as a peace agreement in the Angolan civil war is about to be signed.

At a meeting here this week of the four-nation Joint Commission monitoring the withdrawal of Cuban forces from Angola, the Angolan delegation alleged that South Africa has been sending tons of arms and other supplies to the rebel National Union for the Total Independence of Angola (UNITA).

Such support for UNITA would constitute a clear violation of the December 1988 agreements signed by South Africa, Cuba and Angola providing both for the independence of Namibia and the withdrawal of 50,000 Cuban troops by July of this year.

Until the signing of the agreements, South Africa was the main supplier of UNITA. Angola apparently is concerned that the aid may be resuming as the Cuban troop withdrawal is about to be completed. The United States, the main source of UNITA's arms for the last two years, has agreed to halt military, but not humanitarian, assistance once a cease-fire takes effect. An accord is to be initialed Thursday.

U.S. Assistant Secretary of State Herman Cohen, who attended the Joint Commission meeting as an observer, said in a telephone interview from the Namibian capital, Windhoek, that the Angolan allegations "concern us" and that the United States plans to investigate them.

U.S. officials said they had no evidence of South African supplies going to UNITA since October. "They {the Angolans} are talking about December and January," one official said.

Later, at a meeting of South Africa and Angola, the Angolans backed up their accusations with details on the flights of aircraft and truck convoys allegedly carrying arms to UNITA in December and January across Namibian territory, according to diplomatic sources.

The details reportedly included dates of flights, the number of planes -- allegedly C-130s -- and tonnage of arms.