There were at least three chlorine bomb attacks during February

Two of the attacks were near the city of Falluja, the third near Ramadi, west of Baghdad. Both areas are hotbeds of al-Qaeda militants.

Chlorine gas can cause severe burns to the throat and lungs and can kill after only a few breaths.

There were at least three such attacks in Iraq in February.

The BBC's Hugh Sykes, in Baghdad, describes the use of the gas as an unsettling and possibly ominous development that could greatly increase fear and anxiety.

'Vomiting'

The heaviest casualties were in a suicide attack on Friday evening at the entrance to a housing estate south of Falluja in which six people were reported to have been killed. The injured included at least 27 children.

Officials say that in both attacks the assailants drove dumper trucks containing the bombs at their targets. One of the trucks is reported to have contained a 200 gallon (900 litre) tank of chlorine and explosives.

The third attack was at a checkpoint near Ramadi.

"Approximately 350 Iraqi civilians and six coalition force members were treated for chlorine gas exposure," Lt Roger Hollenbeck of the US-led forces based near Ramadi said, AFP news agency reported.

He said that victims were treated locally "for symptoms ranging from minor skin and lung irritation to vomiting".

Our correspondent says that two of the bombers are thought to have been targeting tribal leaders who have spoken out against al-Qaeda in Iraq.

Easily obtained

Our correspondent says that chlorine is easy to obtain as it is widely used as a cleaner and a water purifier.

After an explosion outside a restaurant near Baghdad last month, as well as the six people killed, dozens of others were left coughing and choking and needing medical treatment.

In February the United States military reported finding a bomb factory near Falluja, where chlorine car bombs were being constructed.

In another attack near Falluja last month, not involving chlorine, more than 35 people were killed when a conventional truck bomb exploded near a mosque where the imam had criticised al-Qaeda in Iraq.

Some tribal leaders in the area have openly complained about al-Qaeda's activities.