Air strikes have targeted Islamic State (IS) fighters near north Iraq's vital Mosul dam after reports of an operation to recapture it fromthe militants.
US media say American fighters and drones were involved, and they were providing air cover to Iraqi and Kurdish troops on the ground.
At least 11 IS fighters were killed, sources in Mosul told BBC News.
The extreme Sunni group, which overran Mosul this summer, has been accused of a new massacre of non-Muslims.
At least 80 men from the Yazidi faith were killed, and scores of women and children abducted, in the village of Kawju (also spelt Kocho) on Friday.
Reports say the men were killed after refusing to convert to Islam. A US drone strike later destroyed two vehicles belonging to the militants.
In another development, German Foreign Minister Frank-Walter Steinmeier visited Iraq, as Western states stepped up aid.
Early reports suggested the aircraft which targeted the dam area could have been Iraqi but several US media outlets, including NBC news, quoted US military officials as saying they were US FA-18 fighter bombers and drones.
"Sources told NBC News the decision to try retaking the dam came after intelligence showed Isis [IS] militants were not yet at a point where they could blow up the installation."
Reports of air strikes also came from residents living near the dam who spoke to AP news agency.
A Kurdish news website, Rudaw, said the air strikes appeared to be the "heaviest US bombing of militant positions since the start of air strikes" against IS last week.
"Kurdish Peshmerga forces are expected to launch a ground assault to retake areas lost to the IS earlier this month," it said.
Another US broadcaster, CNN, reported earlier that a "US and Iraqi military operation" aimed at retaking the dam was scheduled to begin early on Saturday.
The dam, captured by IS on 7 August, is of huge strategic significance in terms of water and power resources.
Located on the River Tigris about 50km (30 miles) upstream from the city of Mosul, it controls the water and power supply to a large surrounding area in northern Iraq.