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

The discovery of the nerve agent, reported today by a team searching for weapons of mass destruction in Iraq, represented the first confirmed find of one of the weapons that the Bush administration cited as initial justification for invading the country last year and toppling the government of Saddam Hussein.

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

He said the round containing sarin had been rigged as a roadside bomb, but was discovered by a U.S. military convoy.

"A detonation occurred before the IED [improvised explosive device] 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."

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 need to track down the origin of the artillery shell and "figure out why it was there and what it might mean in terms of risks to our forces and risks to other people."

Two former weapons inspectors, Hans Blix and David Kay, said the shell was probably a stray weapon that had been scavenged by insurgents and did not necessarily mean that Iraq has large stockpiles of chemical arms, the Associated Press reported.

"It is hard to know if this is one that just was overlooked . . . or if this was one that came from a hidden stockpile," Kay said. "I rather doubt that because it appears the insurgents didn't even know they had a chemical round." The AP quoted Kay as adding that while the discovery of the sarin shows that Hussein did not fully comply with U.N. resolutions, "it doesn't strike me as a big deal."

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.

Iraq told United Nations inspectors it had produced around 800 tons of sarin and thousands of artillery shells, rockets and bombs designed to carry it. But the Hussein government insisted that those stockpiles had subsequently been destroyed in accordance with U.N. Security Council requirements.

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 "the EOD [explosive ordnance disposal] people that went up 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.