Coalition experts are examining dozens of mortar shells found in southern Iraq which could contain chemical weapons.
Danish troops who found them said they showed traces of blister gases - compounds which include mustard gas.
However, US officials played down the find, saying the shells were probably left over from Saddam Hussein's 1980-88 war with Iran.
The coalition has yet to uncover proof that Iraq was still developing weapons at the time of the war last spring.
The 36 120mm shells appear to have been buried for at least 10 years, the Danes said.
Results of more extensive tests should be available in about two days, they said on an official website quoted by the Reuters news agency.
The Danish troops, who serve with the US-led coalition in Iraq and are supported by Icelandic munitions experts, will continue searches for any more weapons buried at the same site, north of Basra.
US military spokesman Brigadier-General Mark Kimmitt said of the shells: "Most were wrapped in plastic bags, and some were leaking."
"We're doing some preliminary tests... to be sure that if they do contain any kind of blistering agent they will be disposed of," he said.
The former regime of Saddam Hussein used blister agents against Iranian soldiers during the Iran-Iraq war.
Chemical weapons were also used to kill about 5,000 Kurds in the northern city of Halabja in 1988.
Before the US-led war to overthrow Saddam Hussein, Iraq said it had destroyed all its chemical weapons, but the alleged continuing threat from weapons of mass destruction were cited by the US and UK leaders as a key reason for the war.
But a nine-month search for stockpiles of chemical, biological or nuclear arms has found no proof of an ongoing weapons programme which could have been used against coalition forces.
On Thursday, a 400-strong team of weapons disposal experts was withdrawn from Iraq but US administration officials insisted their job had been completed.
Blister agents, such as mustard gas, were developed and first used by the Germans in World War I. Italy and Egypt have also used such chemical weapons against enemies.
The agents burn skin, eyes and lungs as they are absorbed, causing large blisters on skin and inside lungs and windpipes.
Effects are delayed for up to 12 hours after exposure, which can allow the agent to cause severe damage before it is detected.