Black-ruled states and liberation movements in southern Africa accused the Reagan administration today of saving the white-minority South African government from impending collapse.

The charges were made at the close of a three-day conference here supporting South Africa's black-ruled neighbors

In a communique, the conference condemned the Reagan administration, Israel and West Germany for nuclear, military, economic and political support of South Africa.

It claimed U.S. relations with Pretoria had enabled South Africa to carry out "policies of aggression, destabilization and economic warfare against the front-line states."

Oliver Tambo, president of the outlawed South African black nationalist movement, the African National Congress, said in an interview that "the present U.S. administration constitutes the single most important new factor in southern Africa. Support from Washington has saved the Pretoria regime from almost certain collapse."

The issue of Namibia (Southwest Africa), a former German colony controlled by South Africa despite a U.N. resolution calling on it to withdraw, dominated the conference, attended by the foreign ministers of Angola, Mozambique and Tanzania and representatives from more than 40 international organizations.

The communique attacked the Reagan administration for linking a South African withdrawal from Namibia to a pullout of the 30,000 Cuban troops from Angola.

Angolan Foreign Minister Paulo Jorge charged that some nations of the western "contact group" seeking a Namibia settlement lacked the courage to risk their ties with South Africa.

"In recent months the contact group has shown a marked incapacity to bypass obstacles mounted by the United States," he told reporters. "Because of this it has failed to resolve the two key questions of Namibian independence: a cease-fire date between SWAPO black nationalist guerrillas and South Africa and a system of free elections acceptable to all sides."