* U.S. rocket strike near Sadr City hospital wounds 20

* Iraq delegation says Iran backs crackdown on militants

* Four U.S. Marines killed in Anbar province

(Adds four U.S. Marines killed in last paragraph)

By Waleed Ibrahim and Tim Cocks

BAGHDAD, May 3 (Reuters) - The U.S. military fired rockets at a target near a major hospital in eastern Baghdad on Saturday, wounding 20 people and damaging several ambulances, the head of the hospital said.

No patients were wounded at the hospital in the Sadr City stronghold of anti-American Shi'ite cleric Moqtada al-Sadr, but 20 people at the scene of the blasts had been hurt, said Dr. Wi'am al-Jawahiri, manager of al-Sadr hospital.

Jawahiri said windows at the hospital were shattered when three missiles hit what the U.S. military in Iraq called a militant "command and control" centre around 10 a.m.

"While I believe the target was not the hospital, we could have been informed before they did such a thing. At least we could have taken some precautions," Jawahiri told Reuters.

The U.S. military said precision-guided munitions were used to destroy the militant facility in Sadr City, where U.S. and Iraqi forces have been battling Shi'ite gunmen loyal to Sadr for several weeks as part of a big government crackdown on militias.

Such weapons could either be rockets fired from launchers on the ground or helicopters.

Colonel Jerry O'Hara, a U.S. military spokesman in Baghdad, said the operation was "time sensitive" and targeted a "command and control" centre that was used to plan attacks against the Iraqi people as well as Iraqi and U.S. security forces.

"We take great care to prevent any collateral damage and will continue to do so. We don't target civilians and regret any casualties," O'Hara said.

Asked why a missile strike was launched so close to the hospital during the middle of the morning, he said:

"The real question should be why these criminal elements seem to always put the Iraqi people at risk by using facilities to coordinate their attacks so close to public places."

A "battle damage assessment" was being carried out, he said.

Reuters Television pictures showed a destroyed building not far from the hospital, one of two main medical facilities in the crowded Shi'ite slum of Sadr City, home to 2 million people.

Several ambulances were badly damaged, along with a number of civilian cars.

MORE CLASHES OVERNIGHT

The latest attack came after a night of more violence in Sadr City. The U.S. military said American and Iraqi forces killed 14 gunmen in battles overnight.

Hospital officials said 14 people had been killed and 25 wounded following the clashes. It was not immediately clear if the dead and wounded were gunmen or civilians.

The U.S. military has been carrying out air strikes on gunmen nearly every day in the militia bastion since fighting erupted more than a month ago.

It says militants have fired more than 700 rockets and mortars at various targets during that period, mostly at the Green Zone government and diplomatic compound.

The U.S. military blames rogue elements of Sadr's Mehdi Army militia for the rocket fire. It accuses Iran of arming, funding and training those militants, a charge Tehran denies.

A delegation from Iraq's ruling Shi'ite alliance was sent to Tehran this week to tell Iran to stop backing Shi'ite militias fighting security forces, members of the alliance have said.

The delegation returned to Baghdad on Saturday. In a brief statement on Iraq's al-Furat television, the head of the delegation, deputy parliamentary speaker Khalid al-Attiya, said Iran supported Baghdad in its fight against militants.

Attiya did not mention the U.S. accusations.

"The delegation saw a positive stance from the brothers in Iran to support the government's efforts in extending the sovereignty of the state and to fight the outlaws," Attiya said.

The U.S. military said this week that "very, very significant" amounts of Iranian weaponry had been found in the southern city of Basra and also Baghdad during an offensive against militiamen in those cities that began in late March.

U.S. military officials had planned to put on display some of the recently captured weapons but decided to let the Iraqis make their own case to Iran first.

Four U.S. Marines were killed by a roadside bomb in the western province of Anbar on Friday, the U.S. military said in a statement. (Additional reporting by Aws Qusay, Writing by Dean Yates, Editing by Ibon Villelabeitia)

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