An artillery shell containing the nerve agent sarin exploded near a U.S. military convoy in Baghdad recently, releasing a small amount of the deadly chemical and slightly injuring two ordnance disposal experts, a top U.S. military official in Iraq said yesterday.

The discovery of the nerve agent, reported yesterday by a team that has been searching for weapons of mass destruction in Iraq since shortly after last year's U.S.-led invasion, marked the first time the team has found one of the types of weapons that the Bush administration cited as initial justification for toppling the government of Saddam Hussein.

But weapons experts cautioned that the shell appeared to predate the 1991 Persian Gulf War and did not necessarily mean that Hussein possessed hidden stockpiles of chemical munitions.

Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, told reporters in Baghdad that the Iraq Survey Group confirmed yesterday that it had found a 155mm artillery shell containing sarin.

Kimmitt said the round containing the nerve agent had been rigged as a roadside bomb, or improvised explosive device, but was discovered by a U.S. military convoy.

"A detonation occurred before the IED could be rendered inoperable," Kimmitt said. "This produced a very small dispersal of agent. . . . Two explosive ordnance team members were treated for minor exposure to nerve agent as a result of the partial detonation of the round."

He said the explosion occurred "in Baghdad a couple of days ago" and that he could not be more specific.

Experts familiar with Iraq's chemical weapons program said the shell was likely a leftover from Hussein's pre-Gulf War stockpile. Iraq acknowledged producing nearly 800 tons of sarin and thousands of sarin-filled rockets and artillery shells between 1984 and 1990.

The experts, including David Kay, the Pentagon's former top weapons hunter in Iraq, said the discovery did not conclusively prove the existence of stockpiles of concealed chemical and biological weapons.

"The question is: Was it part of a cache that contains another ten or twenty of these, or is it one of a kind?" said Raymond Zilinskas, a former U.N. weapons inspector in Iraq and now director of the Chemical and Biological Weapons Nonproliferation Program at the Monterey Institute of International Studies in California. "We have no way of knowing the answer at this point."

Kay, the former leader of the Iraq Survey Group, said the shell was likely one of thousands produced for the Iran-Iraq war. While the Hussein government claimed that all leftover chemical munitions had been destroyed in accordance with U.N. Security Council requirements, it is possible that some were overlooked, hidden or stolen. Before the U.S.-led invasion last year, U.N. weapons inspectors found several empty chemical warheads for rockets and a small number of artillery shells filled with mustard gas.

"This shell may have been scavenged from one of the many munitions storage depots all over the country," said Kay. He said some munitions depots are still not adequately protected.

In Washington, Defense Secretary Donald H. Rumsfeld reacted cautiously to the news, saying he preferred to wait for further testing before commenting on the significance of the discovery.

"We have to be careful," he said during an appearance at the Heritage Foundation in Washington. "We can't say something that's inaccurate." He said investigators have to track down the origin of the artillery shell and "figure out . . . what caused that to be there in this improvised explosive device and what might it mean in terms of the risks to our forces, the risks to other people and any other implications that one might draw."

Kimmitt said the chemical shell was "an old binary type requiring the mixing of two chemical components in separate sections of the cell before the deadly agent is produced." He said the shell, which reportedly was not marked as a chemical round, was designed to work as such a weapon after being fired from an artillery piece, which would cause two chemicals to mix together in flight. But he said the mixing and dispersal of the sarin when the shell is used as a roadside bomb "is very limited."

He noted that "the former regime had declared all such rounds destroyed before the 1991 Gulf War."

It was not immediately clear who had planted the bomb or whether the perpetrators had known that the artillery shell contained a nerve agent.

Kimmitt said he believed that whoever rigged the shell as a roadside bomb did not know it contained chemicals. He said the bomb was "virtually ineffective as a chemical weapon."

Kimmitt said he would leave it to the Iraq Survey Group to determine whether the discovery of the sarin in the artillery shell represents confirmation that Hussein possessed stockpiles of chemical weapons. The 1,200-member Iraq Survey Group had not previously found any of the weapons of mass destruction that U.S. intelligence said Hussein was hiding, although the team found evidence of "program activities" related to such weapons.

Sarin, a liquid nerve agent, causes convulsions, paralysis and asphyxiation. It reportedly was used by Hussein against Iranian forces in the Iran-Iraq war in the 1980s and against Iraqi Kurdish civilians.

Kimmitt said the area in Baghdad where the artillery shell exploded was minimally affected because the binary chemicals that produce the sarin "were not allowed to mix." He said there were "very, very small traces" of the nerve agent as a result of the detonation and that personnel involved in explosive ordnance disposal went to the site and later "showed some minor indications of nerve poisoning." But the exposure was so minor that they were later released, and the area did not need to be decontaminated, he said.

"It was a weapon that we believe was stocked from the ex-regime time, and it had been thought to be an ordinary artillery shell set up to explode like an ordinary IED," Kimmitt said.

Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, told reporters in Baghdad that a roadside bomb made from an artillery shell containing small amounts of sarin was "virtually ineffective as a chemical weapon."