WASHINGTON – The discovery of more than 500 chemical weapons shells in Iraq has heightened concerns at the Pentagon that terrorists in that country could use the old munitions against American soldiers.

“We have recovered enough chemical weapons munitions to make us sensitive to the possible force protection implications these dangerous items present to our forces in Iraq,” a Pentagon spokesman, Lieutenant Colonel Todd Vician, said yesterday in response to a query regarding the recent declassification of an Army National Ground Intelligence Center report that found a large cache of shells from the Iran-Iraq war containing toxic mustard and sarin gas.

Colonel Vician’s statement was a departure from the Defense Department’s initial response. Officials told reporters on Wednesday that the chemical weapons found in Iraq since the conclusion of the official search for weapons of mass destruction were different from what the military went into Iraq to find.

The National Directorate of Intelligence declassified parts of the report after Senator Santorum, a Republican of Pennsylvania, and Rep. Peter Hoekstra, a Republican of Michigan and the chairman of the House Permanent Select Committee on Intelligence, made formal requests this spring. Mr. Santorum said the report “proves that weapons of mass destruction are, in fact, in Iraq.”

The details of where and when the loose ordnance was found have yet to be declassified. But two former intelligence community officials said the Iraqi military left numerous chemical weapons shells in the field of battle at the close of the Iran-Iraq war in 1988.

Even before the American-led invasion of Iraq, the U.N. weapons inspection team led by Hans Blix raised concerns about the whereabouts of the chemical weapons that Iraq used against Iran. “A residue of uncertainty also remains with respect to chemical munitions that were lost, according to Iraq, after the 1991 Gulf war,” a U.N. Monitoring, Verification, and Inspection Commission report from May 30 says. “The Iraq Survey Group quoted conflicting statements of former Iraqi officials, one individual suggesting that some 500 155-mm munitions were retained by Iraq and other officials, insisting that they were actually destroyed.”

That report, to be included in a forthcoming survey of Iraq’s unconventional weapons program, also says some of the chemical shells were likely mixed in with conventional weapons: “Moreover, some chemical munitions filled with chemical warfare agents were marked as standard conventional weapons, which made their identification as chemical munitions problematic, not only for United Nations inspectors and later personnel of the Iraq Survey Group, but also for Iraq.”

The potency of the shells varies, but according to the U.N. report, some of the 1980s-era chemical weapons would be lethal even after nearly 20 years.

No old chemical weapons have yet been rigged to improvised explosive devices used by Iraq’s insurgents. “We have never had an IED utilizing anything but conventional munitions,” a spokesman for Multinational Force Iraq, Major William Willhoite, said.