Western and black African countries failed to reach agreement at the United Nations yesterday on a compromise resolution that would impose a mandatory arms embargo on South Africa.

Another round of informal talks was scheduled for this afternoon, and sources said the U.S. Security Council would probably not meet to vote on an arms embargo before Thursday.

African delegates indicated yesterday that unless a compromise is reached, they will attempt to amend the West German-Canadian resolution now pending before the council to include some of the tougher sanctions vetoed by the United States, Britain and France Monday.

U.S. officials took pains yesterday to explain the vetoes, pointing out that each of the three resolutions in some way went beyond what the Western nations could support at this time.

The first resolution was unacceptable, sources said, because it specifically cited apartheid and South Africa's role in Rhodesia and Namibia in declaring that the policies and actions of Pretoria "constitute a grave threat to international peace and security."

The United States, officials said, feared this would undermine the generally helpful role South Africa has been playing in efforts to solve the Rhodesian and Namibian problems.

The United States vetoed the second resolution, a ban on oil forms of military aid to South Africa, because it went "far beyond a mandatory arms embargo," officials said.

The resolution, for example, required all countries to prevent any companies from "providing any form of direct or indirect assistance to the South African government in its military buildup."

"How could you enforce something like that?" a U.S. official asked. "Any kind of goods - even shoes - could be viewed as indirect assistance."

The U.S. also objected to the resolution's call for a ban on "any cooperation with the South African racist regime in nuclear development."

"Such a ban would remove any ability on our part of influence South Africa to sign the Nuclear Nonproliferation Treaty, and to agree to international safeguards for its nuclear facilities," a U.S. official said.

The third resolution vetoed by the United States would have imposed massive economic sanctions on Pretoria, banning foreign investments in South Africa and calling for curbs on trade.

"This goes well beyond what the situation warrants at this state," the U.S. official said. "We do not favor this kind of sweeping action."