Thirteen suspected insurgents, including three members of al Qaeda in Iraq responsible for the assassination of a Sunni Arab preacher, were killed in a U.S. air strike, the U.S. military said on Thursday.

The strike on Wednesday west of Baghdad came hours after the imam, identified as Abu Bilal and who had been preaching against Sunni Islamist al Qaeda, was killed.

Al Qaeda has vowed to ramp up attacks during the Muslim fasting month of Ramadan, which ends at the weekend, especially against Iraqi and U.S. security forces and against Sunni Arab tribal leaders who have been cooperating with the U.S. military.

Sunni Arab tribal leaders and some Shi'ites in provinces around Baghdad have begun organising their young men into neighbourhood police units similar to those first used in western Anbar province.

The Anbar model has been credited for helping drive al Qaeda fighters out of the province, once the most dangerous area in Iraq but now relatively safe.

The U.S. military said in a statement that an "assassination team" had placed four bombs around Abu Bilal's house, one of which exploded.

"The attackers then entered (Abu) Bilal's house, executed him and wounded his wife," the statement said.

The imam's nephew killed two of the attackers before they fled, the military said. U.S. surveillance teams then tracked them to a nearby field.

"Supporting aircraft engaged the armed men, killing 10 terrorists," the military statement said.

Another three, one of whom was wearing a "suicide vest" packed with explosives, were killed in a later strike.

"Three of the al Qaeda in Iraq members killed in a coalition air strike Wednesday following their assassination of a local imam have been identified as Abu Rami, Ammar Fadhil Kadhim and Fadil Salman," the U.S. military said in the statement.

It said Kadhim was a senior al Qaeda figure in the Abu Ghraib area west of Baghdad and was "known to have killed women and children". Weapons and a suicide belt were found among the dead, it said.