BAGHDAD (Reuters):

A top United states general said yesterday insurgents in Iraq were using crude chemical bombs in a new campaign to create instability, as U.S. and Iraqi forces stepped up a security crackdown in Baghdad.

Two bombs with 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 sicker.

"What they're trying to do is ... adapt in such ways where they can continue to create instability," Lieutenant-General Raymond Odierno, day-to-day commander of U.S. 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 U.S. forces had found chlorine cylinders in a car bomb factory near the rebellious western city of Fallujah 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.

United States 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, the heart of the Sunni Arab insurgency. United States forces in Iraq already number some 141,000.