The United States has suspended payment of more than $194 million in economic assistance to Sudan, its largest recipient of aid in black Africa, because of steady deterioration in the economic and political situation there.

The decision, made late last year and not publicly disclosed, reflects a growing despair among western donors and international aid agencies about how to deal with Sudanese President Jaafar Nimeri, who for the past several years has been bent on the Islamization of his country, apparently without regard to the economic and political cost.

U.S. officials said that the decision was made only after several months of "very high level, across-the-board attention" in the administration, involving Secretary of State George P. Shultz and the National Security Council, and that the Sudanese were informed of it in mid-December.

"There was a consensus but not a happy consensus," one State Department source said. "But there was no other choice."

The decision was not isolated. An economic rescue package put together in 1982 by the International Monetary Fund, the World Bank and western donors and worth $1.5 billion annually in aid and deferred debt payments already had fallen apart because Sudan was $110 million in arrears to the IMF and $264 million behind on its entire 1984 debt service.

Nimeri, regarded as one of the United States' closest African friends, has become a major dilemma for the Reagan administration, which views his country as strategically important to its African and Middle Eastern policies. Nimeri was one of three Arab League leaders who supported the 1978 Camp David Mideast accords and Egypt's peace treaty with Israel a year later.

He also has offered the use of Sudan's air bases and naval facilities to U.S. military forces and assisted in the airlift of thousands of Ethiopian Jews, known as Falashas, to Israel despite the embarrassment last month's disclosure of this secret operation caused his government.

Today, however, Nimeri faces widespread opposition from a broad spectrum of foes, a fast-spreading, Libyan- and Ethiopian-backed insurgency in the south, the influx of a half-million refugees from drought-stricken neighboring states, failing health and a self-inflicted economic mess that is probably the worst in the nearly 16 years he has been in power.

Furthermore, Nimeri has set his mind on imposing the Sharia, or Islamic law, in such a rigorous manner, including the public amputation of thieves' limbs, that the State Department during the past nine months repeatedly has complained publicly about violations of human rights.

Meanwhile, caught in the midst of the renewed Sudanese civil war is the American Chevron Oil Co., which has invested $800 million in exploration, hit a major field and was building an 826-mile pipeline from the oil pumps in the south to the Red Sea coast. After rebels killed three employes last year, Chevron suspended work on the pipeline.

This has angered Nimeri, who had counted heavily on Sudan's first oil exports to generate the foreign earnings necessary to pay off the country's huge debts. He and other Sudanese officials are said to have threatened to cancel Chevron's pipeline and drilling concessions if the U.S. firm does not resume work on the pipeline, Washington Post correspondent Jonathan C. Randal reported from Khartoum.

A Nimeri aide told Randal that the president had "not made up his mind" about the Chevron case and plans to discuss it when he visits Washington next month.

The "Sudan crisis," which has become almost a permanent state of affairs, has provoked a great deal of concern here and in Cairo about how to respond to Nimeri's pressing demands for help on virtually all fronts.

Last March, two deputy assistant secretaries of state for African affairs, Princeton Lyman and James K. Bishop, described the economic and security situation as "grim" in testimony before the House subcommittee on Africa. Since then, both situations have grown considerably worse.

The economic and political woes are so pervasive that the United States and Egypt, Sudan's most important allies, are finding it increasingly difficult to help Nimeri, even though both nations are acutely aware that their withdrawal of support could lead to his fall and a power vacuum that could be filled easily by Libyan- and Ethiopian-backed elements hostile to U.S. and Egyptian interests.

"We're worried," one U.S. official said in frustration. "But to say he is unreformable and to play around with alternatives is dangerous business. He's a friend of the United States, and you can't deny all the things he's done for us: Camp David, his refugee policy . . . . "

After the assassination of Egyptian President Anwar Sadat in 1981, the United States rushed to bolster the Nimeri regime with increased military and economic assistance, fearing that he might be the next target of Moslem extremists or the machinations of Libya's Muammar Qaddafi, who had backed several earlier attempts to bring down Nimeri.

Alexander M. Haig, secretary of state at the time, met with Nimeri during Sadat's funeral in Cairo and sent Robert C. McFarlane, now President Reagan's national security affairs adviser, to Khartoum to assesss the situation. The outcome was a U.S. military and economic aid program starting in 1982 of $200 million to $300 million annually, part of the larger IMF-sponsored package providing Sudan $750 million in western aid and $750 million in debt relief each year.

The package involved massive rescheduling of Sudan's $8 billion to $9 billion accumulated debt and was "almost a precedent in dealing with the LDCs [less developed countries]," a U.S. official said.

The problem was Nimeri's drive to impose Islam, which began in earnest in September 1983. It soon upset Sudan's economic system as he sought to apply Islamic economic principles to taxation and the budget. By mid-1984, Nimeri had plunged Sudan into what one official described as "a major financial crisis" and virtually all western donors were holding back on disbursing their funds.

"They could no longer keep to the IMF targets," another State Department source said. The IMF, in turn, ended its assistance, and the whole package fell apart.

"Then the question was how to put Humpty Dumpty back together again," the source said.

Throughout November and early December, U.S. policy makers dealing with Sudan debated whether to freeze $102 million in economic support funds earmarked for the 1984 fiscal year but still unspent and what to do about the $112 million in similar aid set aside for fiscal 1985.

Around mid-December, they decided that the United States would not free the money because the whole $750 million IMF-sponsored debt relief plan had gone awry, according to the source.

"The pieces weren't working together," he said.

Since then, no economic support funds have been released except for two specific items: $15 million for jute bags needed for the 1985 cotton crop and $4.5 million for a new variety of sorghum seeds.

The "freeze," as State Department officials are calling it, has not affected the U.S. regular economic assistance program to Sudan of $28 million this fiscal year, the $45 million military assistance program or the sending of emergency food to aid refugees. As of December, the United States had sent 82,000 tons of grain worth $20 million and earmarked an additional $50 million for food assistance this year, according to State Department officials.

Further, Sudan is likely to get some of the $235 million the administration is asking for in supplementary assistance for drought-affected African nations.

The problem remains, however, of what to do about the overall economic crisis and whether to continue helping an ally that by all accounts is falling into ever deeper trouble.

Recently Nimeri is said to have told a visitor that he could not accept stringent conditions in return for desperately needed foreign financial aid because of growing public grumbling about shortages.

"Any day now I expect a corporal to march in and shoot me," he was reported to have said.

Like West European donors, the United States is waiting to see the outcome of new negotiations that have just begun between the IMF and Sudan before resuming releasing any furthur economic support funds.

An IMF team visited Khartoum in mid-January, but a spokesman refused to comment on the state of the talks, indicating, however, that they were in a delicate stage.

A Sudanese Embassy spokesman said the Sudan government was "trying very hard" to meet IMF demands for basic economic reforms and cited Nimeri's decision last week to devalue the Sudanese pound by 20 percent. But he said the drought and refugee problems made it "very hard" for Sudan to catch up on its arrears.

Sudan was supposed to pay about $1.5 billion this year just to service its outstanding debt and repay about $264 million in 1984 arrears. But there is no way it can meet these obligations in its current economic condition.

The larger, long-term problem facing the Reagan administration, Egypt and other western donors is whether, and if so, how, to continue aiding Nimeri.

Staff members of the Senate Foreign Relations Committee and the House Foreign Affairs Committee say U.S. policy toward Sudan is likely to come under close scrutiny in the coming months when the administration submits its new foreign aid bill to Congress. The issue, several said, has not become a topic of major concern yet, although the House subcommittee on Africa held hearings last March on Sudan and has tried to block some military aid.

The consensus is that the administration will continue to support the Nimeri regime "come hell or high water," as one staff official put it. "The administration argues a very strong position on the Sudan and Nimeri. It's not going to back away.