Iraqi and US officials said the use of chlorine seemed intended to raise the level of fear and havoc in Iraq, at a time when the United States and Iraq are implementing a new security plan for Baghdad.
|"The enemy is adaptive"|
Lt Col Christopher Garver, US military spokesman
Lt Col Christopher Garver, A US military spokesman, said: "The tactic illustrates the fluid nature of insurgent planning, and the tendency to repeat attacks that draw the most attention and inflict the most suffering".
The US has admitted using White Phosphorus weapons in the Iraqi town of Falluja in 2004 - although it said the weapons were not used against civilians.
On Tuesday, a bomb destroyed a lorry carrying chlorine north of Baghdad, killing at least five people and spewing out fumes that left nearly 140 others, including almost 100 women and children sick, Iraqi police said.
American officials say at least one other attack using chlorine took place in January; 16 people died when a truck carrying a chlorine tank was blown up in Ramadi.
"Has any of the Bush Iraq plans worked other than causing the worst destruction?"
Zaffar Zohair, Islamabad, Pakistan
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"The enemy is adaptive," Garver said, "the enemy wants to win."
Although used widely in water purification and sewage treatment, chlorine is dangerous because it reacts with water to creat hydrochloric acid, which then burns tissue.
Chlorine itself is a green-coloured gas which was used in chemical weapons during World War One; it burns the skin and can be fatal after only a few breaths if inhaled in high concentration.