BAGHDAD ø The U.S. military has discovered the first chemical weapon in Iraq.

U.S. officials said a roadside bomb that contained the nerve agent sarin exploded near a U.S. military convoy traveling outside Baghdad on Monday. They said two soldiers were lightly injured.

The attack could mark the first nonconventional weapons strike against the U.S.-led coalition in Iraq. The Saddam Hussein regime, which was toppled in April 2004, had claimed that all weapons of mass destruction were destroyed in the early 1990s.

"We have to be careful," U.S. Defense Secretary Donald Rumsfeld said. Rumsfeld told the Washington-based Heritage Foundation that tests conducted by the U.S. military could take time to determine the precise chemical in the artillery shell, Middle East Newsline reported.

Officials said the chemical weapon appeared to have come from Saddam's stockpile and that the effect of the explosion was minimal because the agent was used in a roadside bomb rather than being fired by an artillery piece. They said the insurgents probably did not realize that the artillery shell contained sarin.

Officials described the CW round as a "binary chemical projectile." The artillery round has two chambers that keep the chemical components inside separate until they are fired by an artillery piece. After the round is fired, officials said, the rotation of the artillery shell in flight causes the two substances to mix and create sarin. The device releases the agent when it lands and explodes.

When the round is used in an improvised explosive device, officials said, the chemicals fail to mix properly. The result is the production of an extremely small trace of sarin.

"When you rig it as an IED [improvised explosive device], it just blows up and you have minor amounts [of the chemical] going in different directions," Kimmitt said. "It's virtually ineffective as a chemical weapon."

U.S. Army Brig. Gen. Mark Kimmitt, deputy operations director for Multinational Force Iraq, said a U.S. convoy found the 155 mm artillery round rigged as an improvised explosive device. He said the round detonated and spewed a small amount of sarin gas before the explosive ordnance team could render it inert.

Kimmitt said two soldiers were treated for "minor exposure." He said the surrounding area did not require additional decontamination.

Officials said the Iraqi Survey Group would determine if the attack provided any evidence of Saddam's CW or other weapons of mass destruction program. So far, the group has not found any nonconventional weapons.