The U.S. Army said today it had tentatively identified nerve and other chemical agents in drums discovered at a military compound on the Euphrates River.

Commanders cautioned that positive identification of the chemicals by special Fox detection vehicles must be confirmed by more sophisticated analysis. A scientific "mobile exploitation team," or MET, based at Udairi airfield in northern Kuwait was ordered to the suspicious site, but bad weather grounded the team's aircraft until Tuesday morning, Army sources said.

A 101st Airborne Division patrol discovered the chemicals this morning in 11 25-gallon drums and three 55-gallon barrels. After initial field tests indicated toxic chemical agents, two Fox vehicles were dispatched to the compound, which lies on the river east of Karbala. Analyses from both Fox vehicles indicated a high probability of the presence of the nerve agents sarin and tabun and a choking agent believed to be phosgene.

If confirmed, the discovery would provide the first tangible evidence to substantiate Bush administration allegations that Iraq has secretly hidden caches of chemical weapons proscribed under terms imposed after the Persian Gulf War of 1991. But commanders at the 101st headquarters, south of Karbala, cautioned against a rush to judgment.

Several purported discoveries in the last several weeks have proved to be false alarms. On Sunday, several soldiers close to today's site grew nauseous from a substance initially reported as nerve agents; further analysis determined that the suspicious drum contained a weak form of tear gas.

In Washington, Defense Secretary Donald H. Rumsfeld echoed the skepticism of his field commanders by telling reporters, "Let the thing play itself out."

The scientific team is expected to arrive in two CH-47 Chinook helicopters. Procedures call for the team to cordon off the area, conduct field tests, and extract samples.

"We're waiting for the MET team, the real experts, to confirm or refute this," a senior officer said tonight.

The chemicals discovered today were not in warheads or in any obvious weapon form, Army sources said.

The six-wheeled Fox vehicle is a rolling laboratory that tests air, water and ground samples for biological, chemical or radiological hazard. It is not intended to provide definitive answers, but to help combat units avoid and decontaminate hazards on the battlefield. Although its instruments are effective, and its crew of three or four soldiers is trained to operate them, experts said the Fox is not a substitute for laboratory gear operated by scientists.

That is exactly what is coming. The 75th Exploitation Task Force, or XTF 75, includes some of the nation's top weapons scientists, military and civilian, accompanied by Arabic linguists and computer specialists.

The mobile teams carry a complete laboratory in trailer-size shipping containers, with mass spectrometers to identify chemicals at the atomic level and rapid DNA testing for biological toxins.

Phosgene, also known as CG, is a choking agent in use as a chemical weapon since Germany introduced it to the battlefield in 1915. It kills by filling the lungs with fluid in what is sometimes called "dry land drowning."

Sarin and tabun are first generation, or G-series, nerve agents that date from the 1940s. Often called "nerve gas," they are much more common and more effective in liquid form.

Very small quantities, if inhaled or absorbed through skin, kill by paralyzing the central nervous system.

Inspectors from the United Nations Special Commission, or UNSCOM, destroyed some 76 tons of Iraqi tabun and 40 tons of sarin in the 1990s.

But UNSCOM reported in 1999 that it could not account for all the nerve agent known to have been produced, nor for all the thousands of 122mm rockets known to have been filled with sarin.

Iraq is not a signatory to the voluntary Chemical Weapons Convention, but it is bound by the cease-fire terms of the Gulf War to relinquish all such weapons. Because the cease-fire was adopted under the U.N. Security Council's war-making authority, Iraq is the only country in the world under international mandate to give up chemical, biological and nuclear weapons and the long-range missiles capable of delivering them.

That mandate, and assertions that Iraq continued to flout it, was the central basis for the Bush administration's public case for this war.

Gellman reported from New York.

A TV image shows some of the barrels found near Karbala that the U.S. Army said may contain nerve and other chemical agents.A U.S. Army patrol discovered the chemicals in 25-gallon drums and three 55-gallon barrels.