The United States has concluded that Iraq again used chemical weapons in its most recent heavy fighting with Iran and yesterday reiterated strong condemnation of their use.

Secretary of State George P. Shultz had made the U.S. position known Monday in a meeting with Iraqi Foreign Minister Tariq Aziz.

A State Department spokesman said the administration reached its conclusion on the basis of reports issued by doctors who examined Iranians sent recently to West German hospitals and on "other indications available to us" that he refused to disclose.

"Based on this preliminary evidence, we conclude that Iraq used CW, chemical weapons, against the recent Iranian invasion attempt," the spokesman said. "We condemn the use of chemical weapons in violation of international law and convention whenever and wherever it occurs, including this latest instance."

The spokesman refused to confirm that Shultz discussed the issue with Aziz but stressed that the secretary had raised the issue "on repeated occasions" with senior Iraqi officials.

Another State Department official, however, said Aziz left the Shultz meeting "well aware of our concern on this issue," making it clear that Shultz had voiced concern.

The official said he understood that the chemical involved was the irritant and disabling mustard gas, not the more deadly nerve gas known as "Tabun," which the Iraqis reportedly used in February 1983 to halt another big Iranian offensive.

He noted that Iraq was a signatory of the 1925 convention banning use of chemical weapons in warfare.

Aziz refused to comment on the U.S. allegations after meeting Shultz and walked away when a reporter raised the issue.

An Iraqi Embassy spokesman yesterday repeated his government's formal denial that chemical weapons are being used in the 4 1/2-year Persian Gulf war and charged that the allegations are an attempt by Iran "to cover up its huge defeat" in its latest offensive.

State Department officials said the United States plans to "tighten the screws further" on export controls regarding chemical components and machinery used in manufacturing mustard and nerve gases. Last March, after it became known that Iraq had used various gases, the administration banned export of five sensitive chemical components to Iraq and Iran.

One official said, however, that "obviously, there has been some leakage in the system" of export controls.

The official said the administration also plans to consult its European allies and Japan to see what more could be done to tighten similar controls imposed there on the export of chemicals that the warring nations need to produce chemical weapons.

Reemergence of the chemical-weapons issue threatens to complicate relations between Washington and Baghdad just at a time when both sides have been working to expand them after the formal restoration of diplomatic relations. Ties were broken during the 1967 Arab-Israeli war and not restored until last November.

The State Department official, discussing the potential new complication, said the Iraqi use of chemical weapons is definitely "a problem" in current U.S. efforts to improve ties with Baghdad.

The United States still has not named an ambassador to Baghdad, although Iraq has upgraded Nizar Hamdoon, who headed the Iraqi interest section here, to ambassadorial rank. He presented his credentials about two weeks ago and then returned home to serve three weeks at the war front, as all Iraqi ambassadors are required to do.