Iraq is known to have used the blister agent mustard gas from 1983 and the nerve gas Tabun from 1985, as it faced attacks from "human waves" of Iranian troops and poorly-trained but loyal volunteers. Tabun can kill within minutes.
In 1988 Iraq turned its chemical weapons on Iraqi Kurds in the north of the country.
Some Kurdish guerrilla forces had joined the Iranian offensive.
On 16 March 1988, Iraq dropped bombs containing mustard gas, Sarin and Tabun on the Kurdish city of Halabja.
Estimates of the number of civilians killed range from 3,200 to 5,000, with many survivors suffering long-term health problems.
Chemical weapons were also used during Iraq's "Anfal" offensive - a seven-month scorched-earth campaign in which an estimated 50,000 to 100,000 Kurdish villagers were killed or disappeared, and hundreds of villages were razed.
A UN security council statement condemning Iraq's use of chemical weapons in the war was issued in 1986, but the US and other western governments continued supporting Baghdad militarily and politically into the closing stages of the war.