The United States, in a sharp public split with its closest allies, cast a veto in the Security Council today to block a resolution condemning the South African invasion last week of Angola.

The vote was 13-1, with Britain abstaining.

France, Japan, Ireland and Spain joined the other council members from the communist and Third World in voting for a strong condemnation -- which stopped short of any reference to sanctions against South Africa.

American Ambassador Charles Lichenstein, acting in the absence of Ambassador to the United Nations Jeane Kirkpatrick, who is out of the country, explained that "the United States had to vote against a resolution which places the blame solely on South Africa for the escalation of violence which plagues the entire region."

Some diplomats feared that the open break among allies would undermine the five-year long effort by five Western nations -- the United States, Britain, France, West Germany and Canada -- to negotiate the terms of independence for Namibia (South-West Africa), a former German territory administered by South Africa since the end of World War I. The veto was seen as a setback for the original Western plan for Namibian independence.

Its impact was all the more strong because during the last five years the United States and its Western allies have made painstaking efforts to speak and vote with a single voice on issues involving southern Africa at the United Nations.

African perceptions of American intentions were not likely to be as sharply affected, however, because the vote was preceded this weekend by a statement on the U.S. position toward southern Africa by Chester A. Crocker, assistant secretary of state for African affairs. Crocker said the United States will not "choose between black and white" in dealings with racially segregated South Africa.

[Angolan Ambassador Elisio De Figueredo, who asked for the emergency session, called the U.S. veto "nothing short of support of South African racism," The Associated Press reported.]

One American official conceded that the two victors emerging from today's vote were South Africa and the Soviet Union, which portrayed the vote to African nations as an example of the West's inability to resolve southern African questions.

For South Africa, the vote was a tangible result of the new U.S. policy of "neutrality" on African questions, which gives Pretoria a free hand to maintain its tough position on Namibia and on the internal system of apartheid. South Africa reported last week that its troops entered Angolan territory to wipe out Namibian guerrilla position and said its troops are withdrawing.

Lichenstein, while deploring the South African action in Angola, explicitly criticized the "particularly large Cuban force" inside Angola and the Soviet arms and advisers supplied to Namibian guerrillas. These, he said, "fuel the explosive atmosphere of confrontation and violence."

Soviet Ambassador Richard Ovinnikov replied immediately that the American vote was "eloquent proof" of Washington's support for South African racism. He compared the American stand to that of the "raging bull facing a red flag."

Britain explained its abstention by saying the resolution contained charged rhetoric -- such as a reference to South Africa as "racist."

The vote was delayed until late in the evening by negotiations beween French and Angolan officials in which some strong language was dropped from the resolution in return for the French vote.

Britain also discussed further modifications of the resolution. Yet one Western diplomat said, "The British did not really want to come to terms because they were reluctant to split even more sharply with Washington."

In the debate, French Ambassador Jacques Leprette bluntly condemned the South African action as an "unjustified invasion of Angola." He blamed the dangerous situation in southern Africa directly on "the unjustified maintenance in Namibia of a South African presence and the Pretoria government's refusal on mendacious pretexts to accept the implementation of the U.N. settlement plan for Namibia.

The outcome of the debate was a mixed blessing for the Angolans. They won French support, which appeared to be their prime objective. But many diplomats doubted that it had been Angola's objective to demonstrate American isolation.

The Angolans remain anxious to pursue a negotiated settlement in Namibia -- which would rid their territory of both South African troops and guerrillas from the South-West Africa People's Organization (SWAPO).

Today's American veto does not, however, end the pressure on South Africa with respect to Namibia. The special emergency session of the U.N. General Assembly will open here Thursday, devoted exclusively to the Namibian issue.