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.

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.

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.

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Chlorine has been used in at least nine attacks in Iraq since January, mostly in bombings by Al Qaeda 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 plan to fly through Iranian airspace. But the spokesman, Mohammad Ali Hosseini, said the row 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.

The four U.S. soldiers killed Saturday were assigned to Task Force Lightning, the U.S. military said in a statement. A fifth soldier was wounded in the blast.

Diyala province, which lies northeast of Baghdad, 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.

In the past two weeks, U.S. forces cleared two "terrorist safe havens" and surrounding palm groves in Diyala — killing more than 30 suspected militants, capturing 28 and seizing more than 15 weapons caches, the military said in a statement.

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

The Al Qaeda figure was identified as "the gatekeeper to the Al Qaeda 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. Anti-American cleric Muqtada al-Sadr issued the call for a peaceful demonstration to mark the anniversary.

Witnesses said thousands of residents of Baghdad's largest Shiite slum, Sadr City, boarded buses and minivans Sunday for Najaf.

"The faithful should participate in a demonstration in Najaf on April 9, demanding that the occupiers withdraw from our lands. They should carry or wear Iraqi flags," al-Sadr said in a statement released by his office.

On Sunday, Iraqi flags could be seen flying from most houses and shops in Sadr City. 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.

A homicide bomber attacked a police checkpoint near a market in southwest Baghdad, killing a policeman and four civilians and wounding 22 people, two police officials said.

Roadside bombs also killed two Iraqi policemen in separate attacks in the capital and Fallujah, 40 miles west of the capital, police said.

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