South Africa's black-ruled neighbors, bitterly condemning U.S. military support for antigovernment guerrillas in Angola, say the United States has lost credibility as an honest broker in the search for peace in southern Africa.

Leaders of the six countries known as the "front-line states" said after a meeting in the Angolan capital, Luanda, yesterday that the supply of U.S. weapons to rebels of Jonas Savimbi's National Union for the Total Independence of Angola (UNITA) was "a gross and inadmissible interference" in Angola's affairs that had set back peace efforts in the region.

The deliberations of the six nations -- Angola, Botswana, Mozambique, Tanzania, Zambia and Zimbabwe -- added weight to an appeal by the Marxist Angolan government for the United Nations to take over the U.S. role as the principal mediator for an agreement on an independence settlement for the South African-controlled territory of Namibia, formerly known as South-West Africa, Washington Post special correspondent Peter Wise reported.

The southern African leaders deplored what they called an attempt by the Reagan administration to bring Angola into the East-West conflict and said they were particularly offended by the official welcome given to Savimbi when he visited Washington in January, the news agency said.