BAGHDAD, (AP) – The 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 Sunday.

The statement, stamped with al-Sadr’s official seal, was distributed in the Shiite holy city of Najaf on Sunday — 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.”

The U.S. military on Sunday announced the deaths of four American soldiers, killed a day earlier in an explosion near their vehicle in Diyala province northeast of Baghdad. The province has seen a spike in attacks on U.S. and Iraqi forces since the start of a plan two months ago to pacify the capital. Officials believe militants have streamed out of Baghdad to invigorate the insurgency in areas just outside the city.

Separately, a pickup truck loaded with artillery shells exploded Sunday near a hospital south of Baghdad, killing at least 15 people. The blast left a crater 10 yards 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 that 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.

The hospital was slightly damaged by flying debris and shrapnel, but shops and residential buildings bore more damage. Many of those wounded were in their homes at the time of the blast.

Mahmoudiyah is 20 miles south of Baghdad.

Also Sunday, Iran’s state news agency reported that a spokesman for the country’s foreign ministry confirmed that Iran refused to allow Iraqi Prime Minister Nouri al-Maliki’s plane to fly through Iranian airspace. But the spokesman, Mohammad Ali Hosseini, said the dispute was only a technical issue.

“For all flights there is a need for authorization, for which formalities must have been done in advance,” he was quoted as saying.

Members of the delegation traveling with al-Maliki told The Associated Press early Sunday that the plane was diverted to Dubai, United Arab Emirates, where al-Maliki stayed in the airport for more than three hours while his government aircraft was refueled and a new flight plan was filed.

U.S. forces also captured a senior al-Qaida leader and two others in a raid Sunday morning in Baghdad, the U.S. military said.

The al-Qaida figure was identified as “the gatekeeper to the al-Qaida emir of Baghdad” and was linked to several car bomb attacks in the Iraqi capital, the military said in a statement, without naming the captive.

Thousands of Iraqis streamed toward the Shiite holy city of Najaf for a demonstration Monday to mark the fourth anniversary of the fall of Baghdad. Witnesses said thousands of residents of Baghdad’s largest Shiite slum, Sadr City, boarded buses and minivans Sunday for Najaf.

On Sunday, Iraqi flags flew from most houses and shops in Sadr City — as requested earlier in a statement from al-Sadr’s office. Drivers and motorcyclists affixed them to their vehicles. Police escorted convoys of pickup trucks overflowing with young boys waving Iraqi flags, en route to Najaf.

An Iraqi flag was hoisted over a military base in Basra, Iraq’s second-largest city, as Iraqi troops took control of the facility Sunday from British forces. The Shat al-Arab base is the second base transfered to Iraqi control in Basra over the past month.

The Iraqi military ordered a 24-hour vehicle ban in Baghdad on Monday for the anniversary, state television reported Sunday. Al-Iraqiya TV said Brig. Qassim al-Moussawi, spokesman for the Baghdad security operation, disclosed the vehicle ban, which includes motorcycles.

Such bans have been put in place before in an attempt to prevent vehicle bombings.