The U.S. State Department said both Iran and Iraq had used poison gas in the fighting around Halabja and called on both nations to desist immediately.
"This incident appears to be a particularly grave violation of the 1925 Geneva Protocol banning chemical weapons. There are indications that Iran may also have used chemical artillery shells in this fighting," department spokesman Charles Redman said in Washington.
He declined, however, to say what evidence the United States had to implicate the Iranians.
In Tehran, the head of Iran's War Information Ministry told reporters yesterday that Iran "may be forced" to use chemical weapons against Iraq in retaliation.
The official, Kamal Khorazi, also criticized members of the U.N. Security Council for "remaining silent" in the face of the attack on Halabja.
He said that if the "deathly silence" continued, "then maybe we will be forced" to use chemical weapons "to defend ourselves."
The use of chemical weapons such as mustard gas was banned by an international treaty signed in Geneva in 1925.
In Geneva, the International Committee of the Red Cross condemned the use of chemical weapons in the Persian Gulf War.
"In a new and tragic escalation of the Iraq-Iran conflict, chemical weapons have been used, killing a great number of civilians," said the committee, which is the guardian of the Geneva conventions protecting civilians in war.
"The use of chemical weapons, whether against military personnel or civilians, is absolutely forbidden by international law and is to be condemned at all times," it said.
Iranian officials and local survivors said Iraqi warplanes bombed Halabja last Wednesday after it was captured by Iranian troops, dropping canisters with chemical weapons that killed many of its civilian residents.
A spokesman for the Iranian Revolutionary Guards, Alli Shafii, said 5,000 civilians had died in the attacks on Halabja and surrounding towns. There was no way of checking this figure, but reporters who visited the area saw scores of bodies in Halabja and elsewhere.
"The Iraqis, using planes and artillery equipped with chemical weapons releasing mustard gas, cyanide and other types, caused 5,000 innocent people of Halabja and the area to die," Shafii said.
The Iranians say they they had withdrawn their troops from the village shortly before the gas attacks.
Iranian military officials told Western journalists that two Iraqi pilots, whose jets were shot down during the battle, acknowledged that Iraq was responsible for the chemical attack.
And Halabja survivors said in interviews that they were certain the gas attack was launched from an Iraqi warplane.
Iranian officials in Halabja asserted that Iraq's military command staged the gas attack to punish the Kurdish population of the city for its complicity with Iranian forces in seizing control of the city, the Washington Post reported yesterday.
Most of the residents of Halabja were Kurdish civilians. The Kurds, a seminomadic people, have often violently opposed the Iraqi government. Some
Kurds from Iraq are fighting on the side of the Iranians against the Iraqi government.
In Tehran, Iranian medical specialists treating Halabja survivors said the gas cloud contained a mixture of mustard and cyanide gases.
Hundreds of gas victims were evacuated to Tehran hospitals, where Western journalists saw them. Doctors said they were suffering from chemical burns on their skin, eyes and lung tissue.
Hamid Sohrabpour, an internist at Labbfi Nejah hospital in North Tehran, said he had admitted 152 gas victims from Halabja.
"Almost all of our patients are civilian Kurdish people," he said. "They have skin burns all over and in their lungs, and their most difficult problem is breathing."
In 1984, Iran accused Iraq of using mustard gas extensively to repel an Iranian offensive.
At the time, Western diplomatic and military officials said Iraq was producing large quantities of cyanide and mustard gas at insecticide factories. Physicians who examined Iranian victims of the 1984 attacks for the United Nations concluded that they were suffering from the effects of mustard gas and possibly hydrocyanic gas.
The U.N. Security Council in April 1985 condemned the use of chemical weapons against Iranian troops but did not accuse Iraq by name.