President Kenneth Kaunda, regarded as an elder statesman and peacemaker in southern Africa, has bitterly criticized the Reagan administration's decision to give covert aid to Jonas Savimbi's rebel UNITA movement in Angola, saying the United States was "joining with South Africa in destabilizing our region."

Speaking in a recent interview here, the Zambian president said the decision amounted to "a declaration of war" on Angola and "a hostile act" against the informal grouping of African "front-line" states bordering South Africa of which he is currently chairman.

He said it would also entrench the deadlock on discussions about the independence of the South African-controlled territory of Namibia that lies between Angola and South Africa by making it impossible for the Angolan government to get rid of the estimated 30,000 Cuban troops on its soil. Washington and Pretoria have set removal of those troops as a condition for implementing an agreed formula to give Namibia its independence.

Angola needs the Cubans to protect it from UNITA and South African attacks, Kaunda said. "The Reagan administration had made it impossible for its own condition to be met," he added.

U.S. officials have argued that aid to Savimbi's forces is needed to put additional pressure on Angola that would cause its government to send the Cuban forces home. The Reagan administration also has said its policy is aimed at promoting a reconciliation between the Angolan government and UNITA as part of an overall settlement of the various conflicts in southern Africa.

The Zambian president, whose key role in the Namibian negotiations has brought him close to a U.S.-led "contact group" of western mediators, said he had summoned U.S. Ambassador Paul Hare to convey his concern personally to President Reagan.

In a wide-ranging, one-hour interview, Kaunda also expressed fears that South Africa was stepping up attempts to destabilize neighboring black states in order to secure its system of white-minority rule by dominating the whole region.

He said he had received "indirect messages" warning him that South Africa might attack the Lusaka headquarters of the exiled African National Congress, the main black underground movement fighting white rule in South Africa.

Kaunda's criticism of the Reagan administration's decision to help Savimbi is noteworthy because Kaunda was once an admirer and supporter of the anti-Marxist UNITA leader.

He gave aid and sanctuary to Savimbi while the Angolan was fighting with other black independence movements to end Portuguese colonial rule. But when Angola became independent in 1975 under the Marxist-led Popular Movement for the Liberation of Angola, or MPLA, Kaunda recognized the new government and cut his ties with Savimbi.

For more than 10 years Savimbi has been aided in his war effort by South Africa, whose troops sometimes fight alongside his as they raid deep into southern Angola, ostensibly in pursuit of guerrillas of the Namibian independence movement, the South-West Africa People's Organization.

"I think it is appalling that the great American people are led by their president to support a rebellion in a country that is fully independent and internationally recognized," Kaunda said in the interview.

He said the action means the United States is now joining South Africa in invading Angola. "The U.S. has now taken sides with a country that is destabilizing all of us in this region. It is joining in that destabilization."

Part of this destabilization, Kaunda predicted, would probably be the fragmentation of Angola into "a North Korea, South Korea situation," with a strengthened Savimbi gaining control of the southern sector of the country and the MPLA retaining the northern part.

He said he believed South African strategy was aimed at a similar fragmentation of Mozambique, with Pretoria-backed rebels controlling a large northern section of the country while the Marxist government of President Samora Machel retained the southern sector.

Kaunda said he was particularly surprised at the Reagan administration's entrenching of the deadlock on Namibia, because it had declared the achievement of a Namibian settlement a foreign policy priority when it took office. He noted that the Angolan government recently had indicated willingness to compromise on the U.S. demand by removing Cuban troops from southern Angola but retaining them in the north to help defend the capital, Luanda, and the vital Cabinda oilfields.

He said he had discussed these matters "frankly as friends" with U.S. Assistant Secretary of State Chester A. Crocker, but was "unable to fathom the analysis on which they base their actions."