* Blast caused by misfiring rocket kills 18, U.S. says

* Iraqi police blames explosion on truck bomb

* Gunmen kill three U.S. soldiers

By Aseel Kami and Waleed Ibrahim

BAGHDAD, June 4 (Reuters) - Eighteen people were killed in an explosion in Baghdad on Wednesday that Iraqi police said was caused by a truck bomb but which the U.S. military blamed on a misfiring militia rocket.

The huge blast badly damaged 15 houses in the north Baghdad neighbourhood of Shaab, killing 18 civilians and wounding 29, according to the U.S. military. Iraqi police put the death toll at 13 and said 52 people were injured.

Police said four civilians were killed and nine people, including three policemen, were wounded when a parked car bomb targeting a police patrol exploded near a restaurant in the mainly Shi'ite Jadriya neighbourhood of southern Baghdad.

The blasts ended several weeks of relative calm in the capital since a May 10 truce ended weeks of fighting between Iraqi security forces and militants loyal to anti-American cleric Moqtada al-Sadr.

Elsewhere, gunmen killed three American soldiers in the Sunni Arab town of Hawija, 210 km (130 miles) north of Baghdad, the U.S. military said.

A roadside bomb struck an army and police convoy, killing two policemen and one soldier and wounding four policemen near Falluja, 50 km (32 miles) west of Baghdad, police said.

The Iraqi police said a suicide bomber detonated the truck bomb near the house of a senior police officer, Brigadier-General Nadhim Taeih, in the mainly Shi'ite Baghdad suburb of Shaab.

Taeih was not at home but his nephew was killed and his father was seriously wounded in the blast, police sources said.

U.S. BLAMES ROGUE MILITIA

However, the U.S. military said the explosion had been caused by a militia rocket mounted on the back of a truck that exploded prematurely, detonating three or four more rockets.

A U.S. military spokesman, Lieutenant-Colonel Steven Stover, said the truck carried an "improvised rocket-assisted mortar" that could be used to launch rockets on a timer.

"One rocket went off prematurely and set off subsequent explosions," he said.

He blamed the explosion on "special groups" -- rogue units of Sadr's Mehdi Army militia which U.S. officials accuse of receiving funds and training from Iran -- and said it was a coincidence it went off near the police officer's house.

A U.S. military statement on the blast did not mention any militia members among the casualties.

A truck axle lay in the rubble of levelled houses on the street as Iraqi soldiers searched among the debris for bodies.

An old woman wounded in the blast told Reuters Television the explosion was "catastrophic".

"I don't know what we did to deserve this. Is it a punishment from God?" she asked, at Baghdad's al Kindi hospital.

Tabarak Haider, aged about nine, was being treated at the hospital for a stomach wound. She said a truck filled with cartons passed "and when it reached the corner it blew up."

U.S. and Iraqi officials have highlighted an improvement in security in Iraq that has led to sharp drops in violence and in attacks on U.S. soldiers five years after the U.S.-led invasion to topple Saddam Hussein. But U.S. commanders have said that security improvements are fragile and reversible.

Prime Minister Nuri al-Maliki has improved his standing by cracking down on Shi'ite militias in Basra in the south and Baghdad and Sunni Arab insurgents in al Qaeda in the north.

In May, the number of U.S. soldiers killed in Iraq fell to 19, the lowest monthly death toll in a five-year-old war that has claimed the lives of more than 4,000 American soldiers.

Stover said Iraqi police and U.S. soldiers, acting on a tip, found the decaying bodies of between 10 and 12 people in a sewer shaft in the New Baghdad area of the capital on Tuesday.

The bodies, believed to have been in the shaft for about two years, are likely to be victims of sectarian violence, he said.

U.S. President George W. Bush sent 30,000 extra troops to Iraq last year to halt a slide toward sectarian civil war.

Sen. Barack Obama, who declared victory on Tuesday in the race for the Democratic presidential nomination, has pledged to withdraw U.S. troops within 16 months of taking office. Republican rival Sen. John McCain says Democrats' promises to withdraw the 155,000 U.S. soldiers quickly are reckless. (Additional reporting Michael Georgy; Writing by Adrian Croft; Editing by Sami Aboudi)

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