Insurgents in western Iraq set off three chlorine gas car bombs, U.S. forces said on Saturday, weeks after two similar attacks sparked fears of a new campaign to use unconventional weapons in Iraq.

The Friday attacks came a month into a major U.S.-Iraqi security crackdown in Baghdad aimed at stemming sectarian violence that threatens to pitch Iraq into outright civil war.

In Washington, thousands of anti-war demonstrators, some carrying signs reading "U.S. out of Iraq now!" marched toward the Pentagon on Saturday.

Police said about two dozen protesters had been arrested in front of the White House late on Friday, just days ahead of the fourth anniversary of the start of the war.

Anti-war demonstrations were also staged or planned in Los Angeles and Austin, Texas, as well as in Australia, Britain, and Canada.

Australian Prime Minister John Howard, one of Washington's most loyal allies in the war, visited Baghdad on Saturday and told Prime Minister Nuri al-Maliki that Australia would keep its troops in Iraq as long as they were needed.

The Australian Broadcasting Corporation said Howard's plane made an emergency landing after a visit to Australian troops in southern Iraq. The plane's loading bay filled with smoke, forcing it to return to Talil airbase. The cause of the incident was not immediately clear. Howard disembarked safely.

Chlorine gas was widely used in World War One but its use in insurgent attacks in Iraq has particular resonance there. Saddam Hussein attacked Kurdish areas with chemical weapons in the 1980s during the Iran-Iraq war.

The U.S. military said two suicide bombers driving dump trucks carrying chlorine made 350 people ill near the town of Falluja on Friday, and a smaller car bomb near Ramadi also released chlorine, though there were few casualties.

The U.S. statement reported two deaths in one of the attacks but hospital sources said earlier eight people were killed and dozens became ill after the two bombings in the Falluja area.

Two bombs with chlorine killed eight people earlier this year. It causes severe burns when inhaled and can cause death.

The U.S. military said they discovered an al Qaeda car bomb factory last month near Falluja with chlorine tanks.

Anbar, a mainly Sunni Arab province in which Falluja lies, has long been among the most troublesome areas of Iraq for U.S. forces.

HOUSING HIT

Friday's two bombs near Falluja exploded within 40 minutes of each other.

In the first, near the town of Amiriya, two Iraqi police were killed and up to 100 Iraqis showed signs of chlorine exposure, with symptoms ranging from minor skin and lung irritation to vomiting, the U.S. statement said.

Soon afterwards, a suicide bomber detonated a dump truck carrying a 200-gallon (900-liter) chlorine tank rigged with explosives south of Falluja. Around 250 civilians became ill.

Earlier on Friday, a smaller bomb using chlorine detonated at a checkpoint northeast of Ramadi, the capital of Anbar, wounding one U.S. soldier and one Iraqi civilian.

Hospital sources said one of the Falluja attacks targeted a large housing complex, killing six people including policemen, while the second targeted a tribal leader opposed to al Qaeda.

U.S. commanders say a growing number of tribes in Anbar have joined forces to take on al Qaeda, which has hit back with major bombs targeting civilians.

Baghdad was relatively quiet on Saturday, but a suicide car bomb in the west of the city killed two people, including one policeman, and wounded five.

There were no reports of further trouble in Sadr City, a Shi'ite militia stronghold where about 2,000 supporters of radical anti-American cleric Moqtada al-Sadr gathered on Friday to demand U.S. forces withdraw.

Sadr City, a stronghold of Sadr's Mehdi Army militia, is seen as a crucial test of Maliki's plan to crack down on violence by both Shi'ite militias and Sunni Arab insurgents.

(Additional reporting by Mussab Al-Khairalla, Claudia Parsons and Ibon Villelabeitia in Baghdad and Caren Bohan in Washington)