The U.N.'s chief arms inspector for Iraq said Tuesday that experts discovered active mustard gas last month in artillery shells found at an Iraqi ammunitions depot in 1996, raising new questions about more than 500 to 700 similar shells that are still unaccounted for in Iraq.

The inspector, Richard Butler, said at a news conference Tuesday that this case, which he described to the Security Council on Monday, illustrated the recurrent problems encountered by his U.N. Special Commission since it was charged in 1991 with eliminating Iraq's prohibited weapons and the means to manufacture them.Mustard gas is a chemical agent that blisters the skin and lungs, burns the eyes and can cause cancers in the mouth, throat and respiratory tract. Commission officials said Tuesday that they had long suspected that the 155-millimeter shells that Iraq was known to have and that remain unaccounted for were likely to have been filled with mustard gas.

At least four of the 155-millimeter shells were found in 1996 at an ammunition depot in central Iraq, Butler said. The shells were not tested until March of this year, after Iraq argued that they were no longer active .

On Tuesday, Iraq's foreign minister, Mohammed Said al-Sahaf, said at a news conference here that Iraq destroyed all its prohibited weapons in 1991. Carrying on an endless search for banned arms is an "unprecedented injustice to the Iraqi people," he said.

"One of their claims," Butler said of the Iraqis, "was that even if shells like this did exist, they would be useless because the weapons agent inside would be so old that it would have polymerized - it would have hardened and been useless. That's why we drilled holes in a couple of them to see if that were true. And it turned out to be very untrue."