BAGHDAD, Iraq — A roadside bomb containing deadly sarin nerve agent exploded near a U.S. military convoy, the U.S. military said Monday. It was believed to be the first confirmed discovery of any of the banned weapons that the United States cited in making its case for the Iraq war.

Two members of a military bomb squad were treated for "minor exposure," but no serious injuries were reported.

The chemicals were inside an artillery shell dating to the Saddam Hussein era that had been rigged as a bomb in Baghdad, said Brig. Gen. Mark Kimmitt, the chief military spokesman in Iraq.

It appears two chemical components in the shell, which are designed to combine and create sarin, did not mix properly or completely upon detonation, a U.S. official said, speaking on condition of anonymity. Kimmitt, however, said a small amount of the nerve agent was released.

Former U.N. weapons inspector Hans Blix, speaking to the Associated Press in Sweden, said the shell was likely a stray weapon scavenged from a dump and did not signify that Iraq had large stockpiles of such weapons. Kimmitt said he believed that insurgents who planted the explosive didn't know it contained the nerve agent.

U.S. troops have announced the discovery of other chemical weapons before, only to see them disproved by later tests. A dozen chemical shells were also found by U.N. inspectors before the war; they had been tagged for destruction in the 1990s but somehow were not destroyed.

"The Iraqi Survey Group confirmed today that a 155-millimeter artillery round containing sarin nerve agent had been found," Kimmitt said. "The round had been rigged as an IED (improvised explosive device) which was discovered by a U.S. force convoy.

"A detonation occurred before the IED could be rendered inoperable. This produced a very small dispersal of agent," he said.

The incident occurred "a couple of days ago," he said.

The Iraqi Survey Group is a U.S. organization whose task was to search for weapons of mass destruction after Saddam's ouster.

The round was an old 'binary-type' shell in which two chemicals held in separate sections are mixed after firing to produce sarin, Kimmitt said.

Many of the materials used for roadside bombs are believed to have been looted from arsenals after the collapse of the regime in April 2003.

Dispersal of the gas would be far more effective if a shell containing nerve agent were fired from an artillery piece, he said. Kimmitt said he believed it was the first case in which U.S. forces had found an artillery shell containing sarin.

"The former regime had declared all such rounds destroyed before the 1991 Gulf War," Kimmitt said. Two members of a military bomb squad were treated for minor exposure to nerve agent, but none was injured.

It was unclear if the sarin shell was from chemical rounds that the United Nations had tagged and marked for destruction before the U.S. invasion.

Prior to the war, U.N. inspectors had compiled a short list of proscribed items found during hundreds of surprise inspections: fewer than 20 old, empty chemical warheads for battlefield rockets, and a dozen artillery shells filled with mustard gas. The shells had been tagged by U.N. inspectors in the 1990s but somehow not destroyed by them.

In 1995, Japan's Aum Shinrikyo cult unleashed sarin gas in Tokyo's subways, killing 12 people and sickening thousands. In February of this year, Japanese courts convicted the cult's former leader, Shoko Asahara, and sentence him to be executed.

Developed in the mid-1930s by Nazi scientists, a single drop of sarin can cause quick, agonizing choking death. There are no known instances of the Nazis actually using the gas.

Nerve gases work by inhibiting key enzymes in the nervous system, blocking their transmission. Small exposures can be treated with antidotes, if administered quickly.

Antidotes to nerve gases similar to sarin are so effective that top poison gas researchers predict they eventually will cease to be a war threat.

The Bush administration cited allegations that Saddam Hussein had weapons of mass destruction as a main reason for launching the war in Iraq last year.

The Iraq Survey Group, made up of dozens of teams, has been conducting a secretive and largely fruitless weapons hunt across Iraq for more than a year. The survey group combines members of the CIA, the Defense Intelligence Agency, U.S. military Special Forces and others.

The team has run into a number of dead ends. In January, for example, field tests on discovered mortar shells near Qurnah in southern Iraq indicated a blister agent was in the shells. But followup tests indicated that the munitions did not contain the agents, though U.S. officials said Saddam had such agents in the early to mid-1990s.

Officials say there are chemicals associated with certain munitions, such as phosphorous, that can produce false positives. Some field tests are designed to favor a positive reading, erring on the side of caution to protect soldiers.

With no success finding the alleged stockpile of weapons of mass destruction, some of the Iraq Survey Group teams have been redirected to hunting fugitives from Saddam's regime and foreign terrorists believed to be launching attacks in Iraq, military officials have said.

Blix, the U.N. weapons inspector, said in Sweden Monday that before the war, his team found 16 empty warheads that were marked for use with sarin.

Before retiring last year, Blix was the chief of the U.N. Monitoring, Verification and Inspection Commission, known as UNMOVIC, and played a key role in the search for weapons of mass destruction in Iraq. He led the U.N. International Atomic Energy Agency from 1981-1997.

He said it was likely the sarin gas used could have been from a leftover shell found in a chemical dump.

"It doesn't sound absurd at all. There can be debris from the past and that's a very different thing from have stocks and supplies," he said. "I think we need to know more about it."

Blix, whose inspection team didn't make any significant weapons finds during months of searching Iraq before the war, has sharply criticized the United States and Britain for their invasion of Iraq to topple Saddam Hussein's regime.

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Like most people, Blix said he was convinced as late as December 2002 that Saddam had weapons of mass destruction "because we'd seen cat and mouse play" for years by the Iraqis.

But U.N. inspectors had returned to Baghdad the previous month, and as their visits to the best sites provided by foreign intelligence agencies continued to turn up nothing, Blix said he became "more skeptical."

The inspectors were ordered out just before the war began last March, but Blix has said he knew by May "that there were no weapons to be found" because the Americans had interrogated many Iraqis and offered reward money for information — with no results.

The former Swedish foreign minister currently heads a newly created Stockholm-based independent commission on weapons of mass destruction.

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