Iraqi medics are treating dozens of patients poisoned by chlorine after militants targeted civilian areas with trucks loaded with dirty bombs, said Qais Abdulwahab, director of the Kadhimiya Hospital.

On Tuesday, a truck carrying chlorine gas exploded in Taji, just north of Baghdad, killing six people on the spot but also poisoning scores more as the toxic gas spread through the area, overcoming women and children.

On Wednesday, the dirty bombers struck again, in the suburbs of Baghdad, in a less successful attack that nevertheless spread panic.

"The material used is poisonous," said Abdulwahab, comparing the injuries to the internal burns suffered by children who drink chlorine-based cleaning products, "During the explosion it changes into a mist that spreads through the air, causing poisoning in the breathing system, breathing difficulties and acute coughing.

"The use of canisters with something in them is not new, they've tried using regular acetylene tanks to increase the size of the explosive," Abdulwahab added.

"Terrorists are using dirty means," said Brig. Gen. Qassim Moussawi, an Iraqi military spokesman.

Same day, US military officials said ground fire apparently forced the downing of a Black Hawk helicopter. All nine aboard the aircraft were rescued. It would be at least the eighth U.S. helicopter to crash due to hostile fire in the past month.