South Africa has raised new objections to the implementation of a Western plan for holding U.N.-supervised elections in Nambia, calling into question the impartiality of the United Nations and personally attacking Secretary General Kurt Waldheim.

Reporting yesterday to the House Africa subcommittee, U.S. Ambassador Donald McHenry said the latest South African letter to Waldheim regarding the stalled negotiations over Namibia contained a number of allegations against the United Nations that were "distorted" and "unjustified."

He strongly defended the impartiality of the world body and warned of escalating violence in Namibia, just as occurred in Zimbabwe to the point of risking that country's destruction, if South Africa did not accept the U.N. plan soon.

The negotiations, under way now for more than three years, involve South Africa, the United Nations and the five Western powers -- Canada, Britain, France, the United States and West Germany -- which first proposed an election plan for the South African-administered territory in April 1978.

McHenry, who led the five-power negotiating effort before he was named U.S. ambassador to the United Nations in August 1979, said South Africa had still only given its "qualified willingness" to accept U.N.-supervised elections in its Aug. 29 letter to Waldheim.

While he refused to disclose details of the letter, he indicated it contained a direct attack on the secretary general and also on the impartiality of the United Nations because of the General Assembly's acceptance of the national guerila South West Africa People's Organization (SWAPO) as "the sole and authentic representative of the Namibian people."

While the South Africans have attacked the United Nations in the past, this was believed to be the first time they have raised the issue of its impartiality as one of the main reasons for not going ahead with the Western election plan.

McHenry indicated that South Africa was now demanding that the U.N. General Assembly retract its earlier recognition of SWAPO as the sole legitimate representative. "In insisting that SWAPO now be denied that stature it presently derives from its recognition by some in the international community, South Africa is in effect seeking to extract a major concession without committing itself to an agreement," McHenry told the House subcommittee.

The U.S. ambassador said that virtually all of the substantive South African objections to the election plan had been met through concessions on the part of SWAPO and the five African front-line states supporting it.

These include the acceptance of 20 locations where South African troops would be located inside a 62-mile-wide demilitarized zone along the Angolan-Namibian border to make sure no SWAPO troops infiltrated during the elections. The zone would also be monitored by U.N. forces.

Regarding the South African charge of U.N. partiality toward SWAPO, McHenry said the Security Council, which would be in charge of the elections, had never endorsed the guerilla group as the sole representative. He also vigorously defended the impartiality of U.N. peacekeeping operations elsewhere in the world, citing Cyprus and the Middle East as examples.

McHenry said there was now a degree of "urgency" to the settlement plan because changes under way inside Namibia "threaten to unravel the agreements which have already been reached."

He cited as the most significant of these changes South Africa's recent creation of a Council of Ministers inside Namibia composed of Namibian blacks and whites who came to power in South African-supervised elections of December 1978. The elections were boycotted by SWAPO and have not been recognized by any foreign power.

McHenry said the new council was being given "considerable authority" while that of the South African administrator general was being scaled down. He said this was part of a South African two-track strategy that was aimed at preparing an internal settlement of its own within Namibia without SWAPO even while Pretoria continued negotiating with the United Nations.

Noting the escalation of fighting inside Nambia, McHenry said the challenge facing the world community was to find an internationally acceptable solution before the armed struggle reached the proportions of the fighting in Zimbabwe before the settlement there.