Behind the State Department's charge that Iraq has used poison gas against Kurdish rebels is the growing worry that chemical weapons - a "poor country's nuclear bomb" - will spread to the Middle East, Asia and Africa.

Earlier, Iraq used mustard gas and a nerve agent to save itself from defeat in key battles against Iranian troops in the conflict now suspended by a cease-fire. Iran also used chemical weapons, though sparingly.In the Mideast, Israel, Syria and Egypt have chemical warfare capability, according to the London-based International Institute for Strategic Studies.

Libya is believed to be seeking chemical weapons. Ethiopia, victim of poison gas at the hands of Fascist Italy in the 1930s, now possesses chemical weapons of its own.

In Asia, China, Taiwan, North Korea, Vietnam and Burma are in either the "already has" or "is actively seeking" category, Rear Adm. William Studeman, chief of U.S. Naval Intelligence, told a congressional committee earlier this year.

Studeman said the number of nations with chemical weapons has risen sharply in the 1980s. While building a nuclear bomb requires plenty of money, skill and time, chemical weapons can be produced cheaply and quickly. Medium-range ballistic missiles can provide swift and reasonably accurate delivery.

Reacting to the horrors of mustard gas and other horrible killers in World War I, the Western powers in 1925 adopted the Geneva Protocol prohibiting the use of chemical weapons - but not their manufacture and possession.

The Geneva rule held up during World War II and afterward. In 1981, however, the United States charged that "yellow rain" caused by chemical weapons supplied by the Soviet Union to its allies in Laos and Cambodia caused deaths and injuries.

More convincing was evidence of chemical weapon attacks by Soviet troops in Afghanistan in 1982 that caused more than 300 deaths, the United States charged. Moscow dropped the practice.

Only three countries admit to producing chemical weapons. The United States resumed production last year after an 18-year moratorium. The USSR only last year acknowledged having a chemical weapons arsenal. France makes no bones about its chemical weapons.