Washington says it has carried out air strikes against jihadists in Iraq's Sunni Arab heartland, expanding its month-long air campaign and its involvement in the conflict.
Sunday's strikes were a significant escalation for President Barack Obama, who made his political career opposing the war in Iraq and pulled out US troops in 2011.
Previous strikes since Obama launched the US air campaign on August 8 had been mainly in support of Kurdish forces in the north.
US warplanes bombed Islamic State (IS) jihadists around a strategic dam on the Euphrates River in an area that the jihadists have repeatedly tried to capture from government troops and their Sunni militia allies.
"We conducted these strikes to prevent terrorists from further threatening the security of the dam, which remains under control of Iraqi security forces, with support from Sunni tribes," Pentagon spokesman Rear Admiral John Kirby said.
"The potential loss of control of the dam or a catastrophic failure of the dam - and the flooding that might result - would have threatened US personnel and facilities in and around Baghdad, as well as thousands of Iraqi citizens," he added.
It was the first time that Washington had carried out air strikes in support of forces including Sunni Arab militia in the current conflict.
Late last month it gave limited air support to the army, Shi'ite militia and Kurdish fighters in breaking an IS siege of the Shi'ite Turkmen town of Amerli north of Baghdad.
Dams have been a key target for the jihadists, and there has been major fighting around Iraq's largest dam on the Tigris River north of militant-held second city Mosul, which has been a major focus of the US air campaign.
US officials have previously expressed concern about the integrity of both Haditha and Mosul dams, which require constant maintenance as a result of under-investment.
The two dams are important sources of both power and irrigation water for farmers.
Western governments have come under mounting pressure to take strong action against IS, which controls a swathe of neighbouring Syria as well as significant territory north and west of Baghdad.
The jihadist group has carried out a spate of atrocities in areas under its control, some of which it has videotaped and paraded on the internet.
The United Nations has accused IS of ethnic cleansing in northern Iraq, detailing a campaign of mass detentions and executions in Christian, Turkmen and Yazidi Kurdish areas.
The beheading of American journalists James Foley and Steven Sotloff has added to the pressure on Western leaders.
Western governments have voiced mounting concern that nationals who have gone to Syria or Iraq to fight with IS will return home to carry out attacks.
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