Iraqi military forces appeared to be loading poison gas munitions aboard combat aircraft yesterday, potentially threatening use of the weapons in any military engagement with growing U.S. and allied forces in the region, U.S. intelligence officials disclosed.

The officials declined to specify the evidence for their conclusion about Iraq's operations yesterday. But one official said the loading of chemical weapons was "the most plausible explanation for what we have seen."

Iraq is believed to have a supply of several thousand tons of deadly chemical agents, which are occasionally described as "the poor man's nuclear bomb." Its arsenal is known to include bombs, artillery and battlefield rockets capable of carrying mustard gas, a blister agent, and the nerve gases tabun and sarin.

The nerve gases, which disperse swiftly and can cause gruesome, indiscriminate deaths, were invented in Germany before World War II. They can be produced with chemicals that are also used to manufacture fertilizer and pesticides. Iraq obtained extensive battlefield experience with such weapons during its eight-year conflict with Iran, which ended in 1988. After the cease-fire in that war, Iraq "used lethal and nonlethal chemical agents" against its population of ethnic Kurds, according to Central Intelligence Agency Director William H. Webster.

Most of the chemicals were produced with assistance from West Europeans at a heavily protected complex 43 miles northwest of Baghdad, near the village of Samarra. "After several years of experience . . . Iraq's well-established effort now is far less dependent on foreign assistance," Webster told the Senate Governmental Affairs Committee last year.

Although U.S. military forces routinely train in chemical warfare equipment, the standard Army protective suit swiftly dehydrates its wearer, hampers voice communication and lacks any means to dispose of bodily wastes. Military officials say it would be extremely uncomfortable in 100-degree-plus Middle East temperatures.