The State Department is considering an end to U.S. support for the United Nations Environment Program (UNEP), one of the main forums for dialogue between the industrialized West and the Third World. A program official said a cutoff of U.S. aid probably would kill the entire nine-year-old operation.

The voluntary U.S. contribution to UNEP's project fund for fiscal 1982 was budgeted at $7.2 million, down $2.8 million from last year's donation, which made up a third of UNEP's funding.

House and Senate conferees have set the amount at $8.2 million, but Capitol Hill sources said elimination of the entire amount was being considered as part of the Reagan administration's request for an additional 12 percent cut.

UNEP was set up in an agreement among 113 nations in 1972 at the urging of former president Nixon, and is headquartered in Nairobi, Kenya.

It "initiates, stimulates, supports, complements and accelerates" actions within member nations on a wide range of environmental problems, according to an official publication, providing problem assessments, planning, legal drafting and registry services, as well as a technical information clearinghouse. About 500 projects worldwide have been approved for funding.

The U.S. contribution was $10 million a year between 1976 and 1980, about a third of the $30 million project fund, according to Dr. Peter S. Thacher, deputy executive director of UNEP. "We could not survive if the biggest donor suddenly cuts to zero," he said.

Thacher said he had been "frightened" by word of the State Department proposal but had been told by officials there "flatly that it is not official, that it has not been cleared and that flexibility remains."

A spokesman for Eliot Abrams, assistant secretary of state for international organizations, confirmed that assessment.

"I do not feel it was directed against UNEP itself but more that it was just that 12 percent has got to be found someplace," Thacher said in an interview. "I am confident that it is a non-decision so far."

UNEP's governing council last May set a $120 million goal for 1982-83 project funding, a quadrupling of the current level that was roundly condemned by all sides. Western nations, the United States among them, argued that the goal was unrealistically high in light of worldwide economic difficulties. They predicted that no more than $77 million would be collected.

But East Asian nations argued that $120 million was too small given the vast problems of spreading deserts, ocean pollution, soil erosion and deforestation. They asked for a $150 million target. Heated debate lasted a week before the $120 million figure was agreed upon, and only with the proviso that projects be funded as money comes in.