In the volatile Western Iraqi province of Al-Anbar, suicide bombers driving trucks filled with chemicals struck at three different locations late Friday afternoon. U.S. military officials said Saturday at least two Iraqi policemen were killed and 350 civilians and six U.S. troops required medical treatment for exposure to chlorine gas.

A US soldier keeps watch in a Shi'ite enclave of Sadr City in Baghdad, Iraq

In each attack, a suicide bomber detonated a truck packed with explosives and chlorine canisters near police and civilian targets in crowded areas.

U.S. military officials said about 350 civilians required medical treatment. Most were suffering from signs of chlorine gas exposure, with symptoms ranging from minor skin and lung irritations to vomiting.

No group has claimed responsibility for the attacks, but some observers say they bear the hallmarks of al-Qaida in Iraq.

The French Press agency (AFP) reports that a senior Iraqi interior ministry official (Brigadier General Abdel Karim Khalaf) suggested the bombings may have been carried out in retaliation for recent government successes against insurgents in Ramadi.

This is not the first time insurgents have used unconventional weapons. Earlier this year, two separate bombings involving chlorine gas killed several people and sickened dozens more in Baghdad and Ramadi.

In February, the U.S. military said its troops uncovered a car bomb factory near Fallujah that had dozens of propane tanks and ordinary chemicals it believed the insurgents were going to try to mix with explosives.

Meanwhile, in Baghdad, Australian Prime Minister John Howard made an unannounced visit to meet with his Iraqi counterpart, Nouri al-Maliki, and to visit some of Australia's nearly 1,500 troops serving in Iraq.

At a joint press conference, Mr. Maliki said he did not want to put a timeline on Australian withdrawal, while Mr. Howard said his country's troops would remain in Iraq until the job is done.

"I have also told the prime minister that Australia will continue its presence in Iraq to assist in bringing about a situation where the Iraqi people are reasonably able to provide for their own future security," he said.

Most of Australia's troops are stationed in southern Iraq, along with British forces.