The United States has decided to support a mandatory U.N. embargo on all arms sales to South Africa administration sources said yesterday.

It is doing so in an effort to head off a stronger sanction to ban all trade with South Africa, an action U.S. delegates think would be vetoed in the Security Council, one source said.

President Carter is expected to be asked at his news conference today about U.S. policy on South Africa which last week banned 18 anti-apartheid organizations, closed two black newspapers an arrested scores of black leaders and white sympathizers.

The President also is considering unilateral actions that this country might take in protest against South Africa's crackdown, including restrictions on Export-Import Bank loan guarantees to U.S. businesses that export goods to that nation.

However, some sources said they doubted that the President would announce any major unilateral action today. They said restricting or halting the loan guarantees, which total about $200 million, would be a major action.

They said they also doubted that Carter would call on the 6,000 U.S. businesses that trade with South Africa to stop, or the 300 American businesses with plants or offices in South Africa to pull out.

They said, however, he might reduce the number of U.S. diplomats nowtotaling 96, in South Africa.

The House Subcommittee on Africa yesterday approved a resolution censuring South Africa, but stopped short of recommending specific measures in response to the crackdown.

The subcommittee action came shortly after Assistant Secretary for African Affairs Richard M. Moose testified that the administration would welcome a general censure of South Africa but would oppose a resolution recommending retaliatory actions.

Moose said the administration is reluctant to take steps that might lead to the loss of South Africa's promise to withdraw from Southwest Africa (Namibia) and to help move Rhodesia toward majority rule.

U.S. Ambassador to the United Nations Andrew Young is seeking support among U.N. delegates for a proposal saying the supply of arms to South Africa in its current situation constitutes a threat to international peace and security under Chapter 7 of the U.N. charter.

In the 32-year history of the United Nations, Chapter 7 sanctions have been imposed only against Rhodesia in 1966, when the Security Council sought to stop other countries from supplying arms to Rhodesia and importing 10 Rhodesian goods, including chrome ore. Rhodesia had broken away from Britain the year before and was refusing to grant majority rule to blacks. In 1968 the Security Council imposed a full trade embargo on Rhodesia.

Some Western allies of the United States, particularly Britain are worried that invoking Chapter 7 would be what they call a "slippery slope," leading inevitably to full economic sanctions. Britain, which has more than $6 billion in direct investments in South Africa and exports more than $1 billion worth of goods to it, argues that it cannot afford to break off trade.

The United States' view is that the arms sanction would be limited in scope and would not lead necessarily to full economic sanctions.

There is debate about the effect of an arms sanction. Since 1963 the United States and Britain have voluntarily refused to sell arms to South Africa, but France has supplied most of its arms, including Mirage jets, and Israel has supplied missile boats.

One source said the fact that Western nations, including the United States, are for the first time "entering the realm of sanctions against South Africa will have terrific symbolic value, and it will have a tangible effect in time."

In military terms, South Africa is considered a heavily armed country compared with other advanced African nations. It has 362 combat planes, compared with 48 for Rhodesia and 36 for Nigeria. It has 170 tanks, 1,600 armored cars, 230 armored scout cars and 780 armored personnel carrier. In heavy artillery it has 25 105 mm self-prepelled howitzers, 25 added and artillery pieces and 17 90 mm and tank guns. It is said to be self-sufficient in small arms. Its armed forces total more than 55,000 people, including 41,000 in the army, 3,500 in the air force and 5,500 in the navy.

Government sources said the U.N.'s full trade embargo on Rhodesia has had an impact on its economy in the last two years.

Despite the embargo, the United State continued importing chrome ore from Rhodesia until early this year because in 1971 Congress passed legislation saying the President could not prohibit the import of strategic materials from free world countries so long as they were allowed to come from Communist nations.

In March, Congress repealed the legislation and the United States now [WORD ILLEGIBLE] not only boycotting Rhodesian chrome but is requiring stainless steel imports from other countries to be proved free of chorome from RHodesia.

Since the embargo South Africa, which refused to implement it, has been the only legal trading partner of Rhodesia but other nations are said to have violated the ban secretly. Still the embargo, combined with other factors - Rhodesia's high military spending a 15 per cent inflation rate and weak metals prices - has contributed to the stagnant economy that Rhodesia acknowledges it now has U.S. sources said.