(Adds U.S. deaths paragraph 2)

By Paul Tait

BAGHDAD, Oct 5 (Reuters) - A U.S. air strike killed about 25 suspected Iraqi militants linked to Iranian-backed Shi'ite militias on Friday and another 12 al Qaeda fighters were killed in separate raids, the U.S. military said.

It said it had lost four soldiers across Iraq. Roadside bombs killed one soldier in the town of Baiji north of Baghdad and two more in the capital. Another U.S. soldier was killed by small arms fire in Baghdad on Thursday.

U.S. troops said they were engaged in a heavy firefight west of Baquba, capital of volatile Diyala province north of Baghdad, during a dawn raid against a commander it said was linked to the Iranian Revolutionary Guards' elite Qods force.

The U.S. military also said it had killed 12 suspected al Qaeda in Iraq fighters during separate strikes in Baghdad and Yusifiya north of the capital on Friday.

In Salahuddin next to Diyala, police said Sheikh Muawiya Jebara, a Sunni Arab tribal leader who had worked with U.S. forces in forming local police units to fight Sunni Islamist al Qaeda, had died of wounds suffered in a bomb attack on Thursday.

U.S. commanders in Iraq have often accused Shi'ite Iran of training and arming Shi'ite militias in Iraq and supplying them with weapons, including rockets and roadside bombs, by far the biggest killers of U.S. troops in Iraq.

Tehran denies the charge and blames the sectarian violence in Iraq, in which tens of thousands of Iraqis have died, on the 2003 U.S.-led invasion to topple Saddam Hussein.

"HOSTILE INTENT"

In the Baquba operation, support aircraft were called in when U.S. soldiers came under attack from militants, with one insurgent thought to have an anti-aircraft weapon.

"Perceiving hostile intent, support aircraft engaged, killing an estimated 25 criminals and destroying two buildings," the U.S. military said in a statement.

Police and hospital sources said 25 people were killed and another 35 wounded in the U.S. air strike in the village of Jezan al-Imam near Khalis, a town northwest of Baquba. They said four houses were also destroyed.

Police sources said most of the dead were men, disputing Iraqi television reports that women and children were among civilian casualties.

The operation early on Friday targeted what the U.S. military called a "special groups" commander, a term it often uses to describe militants it says are linked to Iran.

"Intelligence indicates he was responsible for facilitating criminal activity and is involved in the movement of various weapons from Iran to Baghdad," the statement said.

It did not say whether the man was among those killed.

While not specifically linking the man to the Mehdi Army militia loyal to fiery anti-American cleric Moqtada al-Sadr, the U.S. military said it welcomed Sadr's pledge in late August to suspend all Mehdi Army operations for up to six months.

"We will not show the same restraint against those criminals who dishonour this pledge by attacking security forces and Iraqi citizens," the statement said.

In Samarra in Salahuddin province, an Iraqi government security source said Iraqi soldiers and police killed 18 gunmen and arrested 38 during raids targeting al Qaeda fighters on Thursday and Friday.

The U.S. military began a security crackdown in Baghdad in mid-February which then spread into other volatile areas across Iraq using a "surge" of 30,000 extra U.S. troops in support of thousands of Iraqi security forces.

The crackdown is aimed at Sunni Islamist al Qaeda and Shi'ite militias and has been credited for a significant drop in military and civilian casualties in recent weeks.

South of the capital, the U.S. military said it was investigating the deaths of three civilians shot by U.S. troops near a checkpoint manned by local tribal police on Thursday.

Local residents said six people were killed but the military said it was only investigating three deaths in Abu Lukah.

Forming what the U.S. military calls "concerned citizens" into local tribal police has been praised as a rare security success story in Iraq and proof that President George W. Bush's "surge" strategy in Iraq is working.

(Additional reporting by Aws Qusay and Dominic Evans in Baghdad)

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