The future of an international food aid organization sponsored by Western nations and the Organization of Petroleum Exporting Countries is in doubt in the wake of a disagreement on how the financing burden is to be shared by the two groups.

The organization, the International Fund for Agricultural Development (IFAD), was set up in 1974 to try to increase the productivity of small farmers in the poorest countries.

Under discussion is a three-year replenishment of funds of approximately $600 million, about half of the prior three-year commitment of $1.1 billion. The United States is the sole holdout against a funding formula under which the Western group would increase its share of IFAD's budget from 58 percent to 60 percent.

In the wake of the disagreement on that shift at a meeting in Rome May 16-17, a Reagan administration interagency group this week discussed pulling the United States out of IFAD, considered one of the most efficient of the aid agencies.

In a telephone interview yesterday, the leading American negotiator for IFAD laid the blame for the deadlock on OPEC, which he said "is refusing to put a firm commitment on the table."

Richard Derham, an assistant administrator for the Agency for International Development, said that more than a year ago the United States offered to provide 17 percent of the IFAD bill up to a maximum of $150 million for three years, if OPEC agreed to accept a 50-50 share of the financing burden with the Western countries.

The original understanding, Derham said, was that OPEC would share the financing "as close as possible" to exact equality -- the same basis on which they share power in IFAD.

"It's clear at this point after five meetings with OPEC over 18 months they they have not come up with a number, and we are not prepared to bargain with them" until they do, Derham said.

But others said the United States was applying too rigid a rule. "If the United States continues to hold up new replenishments, IFAD will be unable to fund new projects at a time when many African countries are crying out for help," John Tucker of Bread for the World, a citizens' lobby group, said yesterday.

Twenty Western countries -- members of the Organization for Economic Cooperation and Development -- and 12 OPEC nations have been contributing to IFAD, with the OECD group putting up 58 percent of the total, and OPEC 42 percent. The original ratio, when IFAD began, was 57 to 43 percent.

More than a year ago, the OPEC nations, which have seen their incomes decline sharply, proposed that the next three-year replenishment beginning this year be on a 60-40 basis. All the Western nations except the United States have agreed to that split. The American delegation to last week's Rome meeting, led by Derham, had explicit instructions that a change from the 58 to 42 ratio was not negotiable.

The total difference for the OECD nations would be only $12 million -- the difference between $360 million as a 60 percent share and $348 million as a 58 percent share. The extra cost to the United States would be $3.5 million. "Obviously," said a member of the U.S. delegation to the Rome IFAD meeting, "this is peanuts. But the U.S. wants to establish the principle that OPEC's share of the burden should not be ratcheted down."

The other OECD nations offered to provide the $3.5 million but the United States has rejected this compromise.

After bitter denunciations of the U.S. position at a plenary session in Rome last week, IFAD president Idriss Jazairy proposed that the meeting be suspended, but not adjourned, pending further negotiations. Jazairy is scheduled to come to Washington in a few days to meet with AID Administrator Peter McPherson and Vice President George Bush, a strong supporter of IFAD within the Reagan administration.