BAGHDAD (Reuters) - About 25 suspected Iraqi militants were killed in an air strike on Friday during a raid targeting a "special groups" commander working with Iran's Revolutionary Guards, the U.S. military said.

U.S. troops were engaged in what was described as a heavy firefight west of Baquba, capital of volatile Diyala province north of Baghdad, during a raid around dawn against a commander it said was linked to Iran's elite Qods force.

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 in Abu Lukah village near Mussayab on Thursday.

In the Baquba operation, support aircraft were called in when U.S. soldiers came under attack from militants firing assault rifles and rocket-propelled grenades, 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.

U.S. commanders in Iraq have repeatedly accused Shi'ite Iran's Revolutionary Guards 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.

The operation early on Friday targeted what the U.S. military described as 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 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 dishonor this pledge by attacking security forces and Iraqi citizens," the statement said.

In Samarra in neighboring 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, which came into full force in June, 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.

Forming what the U.S. military calls "concerned citizens" into local tribal police, along the lines of a model first used in western Anbar province, has been praised as a rare security success story and proof that President George W. Bush's "surge" strategy in Iraq is working.

The U.S. military said the civilians killed in Abu Lukah on Thursday were near a "concerned citizens" checkpoint, but few other details were available.

Local residents said six people were killed but the U.S. military said it was only investigating three deaths.

The "surge" of 30,000 extra U.S. troops is meant to buy time for Iraq's leaders to meet legislative benchmarks set by Washington aimed at reconciling majority Shi'ite and minority Sunni Arabs who were dominant under Saddam.