A top US general said this evening insurgents in Iraq were starting to use using crude chemical bombs.
Two bombs using chlorine gas have killed up to 11 people this week. The blasts, one in Baghdad and the other north of the capital, caused toxic fumes that have made scores more sick.
"What they're trying to do is ... to adapt in such ways where they can continue to create instability," Lieutenant-General Raymond Odierno, day-to-day commander of US troops in Iraq, told reporters at the Pentagon in a live link-up.
"That's what they're doing, especially with these chlorine IEDs (improvised explosive devices)," he said, adding US forces had found chlorine cylinders in a car bomb factory near the western city of Falluja on Tuesday.
Chlorine gas was used as a weapon in World War One but its use in guerrilla attacks in Iraq has particular resonance for Iraqis. Saddam Hussein used chemical weapons on Kurdish areas in the 1980s during the Iran-Iraq war.
US President George W. Bush is sending 21,500 more troops to Iraq in an effort to drive militants out of Baghdad and to try to stabilise Anbar province, heart of the Sunni insurgency. US forces in Iraq number some 141,000.
In the city of Ramadi, capital of Anbar province, US forces killed at least 12 alleged insurgents and wounded three others in a six-hour clash involving air strikes, the US military said.