The World Bank said yesterday the United States has committed to make an annual $950 million contribution to its subsidized loan affiliate, which will ensure at least an $11.5 billion program for the poorest nations over the next three years.

Moeen Qureshi, senior vice president for finance, announced after a meeting of deputy ministers in Paris that "we have achieved a major breakthrough in these negotiations." He said the agreement will ensure that the eighth replenishment of resources of the International Development Association (IDA-8) will be near its $12 billion target.

But informed sources on Capitol Hill immediately suggested that a $950 million annual U.S. appropriation for IDA was unrealistic in view of across-the-board budget cuts being made in all foreign-aid appropriations.

"I suspect that the administration is just going to say that they tried to get a generous IDA replenishment," said one Republican on the Hill. "If they come in and say this is a sacred commitment, they will be surprised."

But a senior Reagan administration official said in a telephone interview that other nations had been informed in Paris by Assistant Treasury Secretary James W. Conrow that the $950 million pledge is contingent upon agreement by Congress. He conceded that it will have hard sledding in both houses, but added, "We're serious about it."

In the past few years, the United States has contributed 25 percent of IDA funds, which are given interest-free to poor nations over a 50-year term, subject to small annual service charges. A 25 percent share of an $11.5 billion program works out to $958.3 million annually.

That would be substantially more than the $750 million that the United States has given as its annual share of the current $9 billion IDA-7 program. Congress has appropriated only $680 million for IDA for fiscal 1986 (plus $72 million for a special African loan program), and another 10 percent shortfall appears inevitable for fiscal 1987.

In effect, the administration appears to be planning to ask for about $300 million a year more for IDA than is currently being appropriated. The total would be closer to what it was during the Carter administration.

In earlier negotiations on IDA-8, the bank set a $12 billion goal, with the extra $3 billion increment over IDA-7 earmarked for Africa. Half of the $3 billion would duplicate an existing $1.5 billion special fund; the other half would be a new pool of money to match another special program announced in March by the International Monetary Fund.

Until yesterday's announcement, Treasury Secretary James A. Baker III had said only that the United States would back an IDA-8 program between $10.5 billion and $12 billion. At the Tokyo economic summit, President Reagan had joined other leaders in approving "an early and substantial replenishment" for IDA-8, recognizing Africa's critical dependence on this source of funds.

In announcing what he termed the success of the negotiations, Qureshi said the officials present had agreed that a minimum of 45 percent of the total funds should be allocated to sub-Saharan Africa, and that two other large recipients, India and China, should get about 30 percent.

Qureshi also said that the donor nations had agreed to shorten the traditional 50-year IDA maturity, and to consider ways of reviewing which countries had made enough progress to lose their IDA eligibility.