WASHINGTON, 23 February 2007 — Insurgents are devising new tactics to sow chaos in Iraq, fielding specialized cells to shoot down helicopters and adding chlorine bombs to its arsenal, senior US military officials said yesterday.

Insurgents have blown up trucks with tanks filled with chlorine twice this week, leaving scores of people sickened by toxic fumes in the latest twist on the daily mayhem in Iraq.

A raid on a big bomb-making factory near Fallujah on Tuesday uncovered chlorine tanks and fertilizer along with a pickup truck and three cars being turned into bombs, they said. “That’s just another way they’re trying to adapt to cause some sort of chaos here,” said Lt. Gen. Raymond Odierno, the second ranking US commander in Iraq.

At the same time, Odierno said cells affiliated with Al-Qaeda in Iraq are believed to be stalking US helicopters, a vital cog in the US military machine. Two or three of the eight helicopters that have been downed since Jan. 20 appeared to have been ambushed, he said.

“I think they’ve probably been trying to do this for a long time, but my guess is we have a cell out there that’s somewhat effective,” he said.

The shoot downs come as the US military’s reliance on helicopters to move troops and support those on the ground is surging.

In 2005, US military helicopters in Iraq flew 240,000 hours. Last year they flew, 334,000 hours and the current rate they’ll fly 400,000 hours this year, Odierno said. “We realize that effective use of helicopters is essential to operational success. And we are using all means at our disposal to protect our aircraft,” he said.

Of the eight helicopters that crashed, at least six are believed to have been brought down by enemy fire. In all, 27 people have died in the crashes.

Insurgents have used combinations of different types of heavy caliber small arms fire to foil heat-seeking countermeasures, military officials said.

The military also is studying how to cope with the sudden emergence of chlorine bombs. The site raided near Fallujah had five buildings filled with munitions, propane tanks, ordinary chemicals and other materials for making car bombs, officials said.

Insurgents have tried over the past couple of years to use chemicals to make car bombs more lethal, and using chlorine cylinders was “just another way to do it,” Odierno told reporters here via video link from Iraq.