US military officials say they are concerned about recent attacks by insurgents in Iraq involving poisonous chlorine gas.
There have been three chlorine bombs since 28 January
The warning came after a pick-up truck carrying chlorine gas canisters rigged with explosives blew up in south-west Baghdad, killing two Iraqis.
Many others were wounded in the attack. It was the third such bombing in Iraq this month.
Chlorine burns the skin on contact and can be fatal after a few breaths.
After Wednesday's bombing, a spokesman for the US military said chemical attacks appeared to be a fresh tactic by the insurgents, reflecting a desire to cause mayhem.
He predicted there would be more copycat attacks by other militant groups.
On Tuesday a bomb blew up a truck carrying chlorine north of Baghdad, killing at least five people and emitting fumes that made more than 150 others ill.
Three weeks ago, a truck carrying explosives and a chlorine tank blew up in Anbar province. More than 12 people were reported to have been killed. An Iraqi military spokesman, Brigadier General Qassim Moussawi, said militants were now using "dirty means".
The BBC's James Westhead in Washington says the bombings do not seem to reflect any new technical capability.
He says chlorine may have been used because it is present in large quantities in Iraq to deal with the country's poor water sanitation.