RENEGADE cleric Muqtada al-Sadr urged the Iraqi army and police to stop cooperating with the United States and told his guerrilla fighters to concentrate on pushing American forces out of the country, according to a statement issued yesterday.

The statement, stamped with al-Sadr's official seal, was distributed in the Shiite holy city of Najaf yesterday - a day before a large demonstration there, called for by al-Sadr, to mark the fourth anniversary of the fall of Baghdad.

"You, the Iraqi army and police forces, don't walk alongside the occupiers, because they are your archenemy," the statement said. Its authenticity could not be verified.

In the statement, al-Sadr - who commands an enormous following among Iraq's majority Shiites and has close allies in the Shiite-dominated government - also encouraged his followers to attack only American forces, not fellow Iraqis.

"God has ordered you to be patient in front of your enemy, and unify your efforts against them - not against the sons of Iraq," the statement said, in an apparent reference to clashes between al-Sadr's Mahdi Army fighters and Iraqi troops in Diwaniyah, south of Baghdad. "You have to protect and build Iraq."

Meanwhile, the US military announced the deaths of four American soldiers, killed on Saturday in an explosion near their vehicle in Diyala province northeast of Baghdad. The province has seen a spike in attacks on US and Iraqi forces since the start of a plan two months ago to pacify the capital. Officials believe militants vacated Baghdad to invigorate the insurgency in areas outside the city.

Separately, a pickup truck loaded with artillery shells exploded yesterday near a hospital south of Baghdad, killing at least 15 people. The blast left a crater 10 meters wide, the Iraqi military said.

Three mortars sailed into houses in eastern Baghdad, sending six people to the hospital with breathing difficulties from a possible chemical agent, police said.

Doctors said the victims' faces turned yellow and they were unable to open their eyes. One hospital official said the chemical was chlorine and the victims were expected to recover.

Chlorine has been used in at least nine attacks in Iraq since January, mostly in bombings by al-Qaida in Iraq.

The bombing in Mahmoudiyah involved a pickup truck parked next to the city General Hospital, an Iraqi army officer said. He spoke on condition of anonymity because he was not authorized to speak about the matter. Other reports said the explosion was a rocket attack.

At least 26 people were wounded, he said.

Hours later, five burned and mutilated bodies remained scattered at the scene. Most of the dead were technicians who worked at auto repair shops nearby, officials said.