U.N. weapons inspectors uncovered 11 empty chemical warheads in "excellent" condition in Iraq on Thursday -- materials the U.N. said were not included on a "complete" list of weapons Baghdad previously said it had in its possession.

Iraq insisted that it had reported the rockets, which it said were old and never used for chemical weapons.

Also Thursday, inspectors searched the homes of two Iraqi scientists in Baghdad for the first time. One of the them, a physicist, left with inspectors, but it was unclear if there was any connection between the home search and the discovery of the munitions.

Debate immediately began about whether the warheads constituted a material breach under U.N. Resolution 1441.

The Bush administration insisted that Iraq was violating the resolution regardless of whether the warheads are in violation.

State Department spokesman Richard Boucher said the inspectors themselves have indicated that Iraq has failed in a number of areas to cooperate fully with U.N. Security Council requirements.

"There's no point in continuing forever, going on, if Iraq is not cooperating," Boucher said.

White House spokesman Ari Fleischer said the administration was assessing the warhead discovery and would be deliberate about reacting to it.

The resolution stipulates Iraq must declare any banned weapons, their locations and related materials. Any false statements or the failure to cooperate "shall constitute a material breach," which could be a trigger for war.

U.S. officials, speaking on the condition of anonymity, said the discovery may not amount to a "smoking gun" unless some sort of chemical agent is also detected. Key questions about the find are whether any chemical weapons were ever loaded into the ordnance, and, if so, when, officials said. Serial numbers on the rockets should tell inspectors where and when they were made.

The 122 mm warheads were found in bunkers built in the late 1990s at the Ukhaider Ammunition Storage Area, 75 miles south of Baghdad, Hiro Ueki, the inspectors' spokesman in Baghdad, said in a statement. The team examined one of the warheads with X-ray equipment and took away samples for chemical testing, the statement added.

Ueki told The Associated Press the shells were not accounted for in Iraq's declaration. "It was a discovery. They were not declared." He also said a 12th warhead was also found that needed further evaluation.

But Lt. Gen. Hossam Mohammed Amin, the chief Iraqi liaison officer to the inspection teams, said they were short-range shells imported in 1988 and mentioned in Iraq's report. He expressed "astonishment" over what he called "no more than a storm in a teacup."

Amin said the inspectors found the munitions in a sealed box that had never been opened and was covered by dust and bird droppings.

"When these boxes were opened, they found 122 mm rockets with empty warheads. No chemical or biological warheads. Just empty rockets which are expired and imported in 1988," Amin told reporters, adding similar ordnance was found by U.N. inspectors in 1997.

David Albright, a former nuclear weapons inspector in Iraq, said the discovery would represent a violation "if Iraq knew that these warheads existed and they are for chemical weapons."

Another former inspector said that at one time, Iraq had thousands of warheads filled with chemical agents.

"Trained chemical inspectors should be able to tell pretty easily whether the rockets discovered on Thursday are designed to be filled with chemical agents," said Terry Taylor, who heads the Washington office of the London-based International Institute for Strategic Studies.

On Dec. 7, a chemical team secured a dozen artillery shells filled with mustard gas that had first been inventoried by earlier inspectors in the 1990s. Those were the first weapons of mass destruction brought under inspectors' control in the current search, which began in November.

Chief inspectors Hans Blix and Mohamed ElBaradei have said Iraq's weapons declaration is incomplete -- failing in particular to support its claims to have destroyed missiles, warheads and chemical agents such as VX nerve gas.

The United States, which has begun a heavy military buildup in the Persian Gulf, has threatened war on Iraq if it is found to be hiding banned weapons programs. The Iraqi government says it no longer has any chemical, biological or nuclear weapons and submitted a 12,000-page declaration to the United Nations last month that it said proved its case.

During the search at the Iraqi scientists' homes, the inspectors escorted one of them to a field to examine what appeared to be a man-made mound of earth. The scientist, who carried a box of documents as he left his house, was then taken to the inspectors' hotel along with the documents and Iraqi officials.

Amin said the inspectors also asked to speak privately at their hotel with two other scientists linked to Iraq's weapons programs Thursday, but the scientists refused to be interviewed without Iraqi officials present. The inspectors did not interview the two scientists, whom Amin did not identify.

Blix and ElBaradei have stepped up demands that Iraqi improve its cooperation. Iraqis "need to be more active ... to convince the Security Council that they do not have weapons of mass destruction," Blix said, adding that the alternative is "the other avenue ... we have seen taking shape in the form of military action."

The homes searched Thursday were those of physicist Faleh Hassan and his next-door neighbor, nuclear scientist Shaker el-Jibouri, in the Baghdad neighborhood of al-Ghazalia.

It was the first time the inspectors have searched private home since they resumed their work. The team searched the homes for six hours, with experts seen going through documents at a table set up near Hassan's front door and having an animated discussion with Iraqi liaison officials.

Afterward, Hassan -- who is director of al-Razi, a military installation that specializes in laser development -- drove with the inspectors and Iraqi officials about 10 miles west of Baghdad to an agricultural area known as al-Salamiyat. There, Hassan, two inspectors and a liaison officer walked to a bare field and examined the mound of earth for about five minutes.

Inspectors did not speak to journalists and it was not clear why they were interested in the mound. An Iraqi official later said the field was a farm that Hassan sold in 1996.

After the visit, a visibly angry el-Jibouri told reporters the inspectors spent two hours in his home -- and cordoned it off for much longer -- looking into everything, "including beds and clothes."

"This is a provocative operation," he said. "They did not take away any documents but they looked at personal research papers."

The Associated Press contributed to this report.