An Iraqi security official said on Sunday Kurdish forces have taken over parts of the country's largest dam, which was captured by the Islamic State (Isis) extremist group earlier this month.

General Tawfik Desty told the Associated Press that peshmerga forces backed by Iraqi and US warplanes started the operation to retake Mosul Dam early on Sunday.

Desty, a commander with the Kurdish forces at the dam, which was seized on 7 August, said they now control the eastern part of the dam and that fighting is still underway.

The US launched airstrikes against Isis fighters more than a week ago, in a bid to halt its advance across the north. The extremists control vast swaths of Iraq and neighboring Syria.

Kurdish forces supported by American warplanes have mounted an offensive to retake Iraq's largest dam, a formidable hydroelectric complex critical to both power supplies and irrigation in the region, from jihadi fighters, as reports emerged of another grisly episode of mass slaughter perpetrated by the extremists in a village in northern Iraq.

US central command said on Saturday that fighter jets and drones had destroyed or damaged four armoured personnel carriers, seven armed vehicles, two Humvees and an armoured vehicle.

The US engagement is aimed at helping the Kurds turn the tide against the Isis extremists who have swarmed through parts of northern Iraq from bases in Syria, seizing towns and cities and slaughtering opponents indiscriminately.

Villagers said Isis militants drove into a settlement on Friday, rounded up men and teenage boys, lined them up and shot them. The reports came from several men who survived the massacre in Kocho. Senior Kurdish official Hoshyar Zebari said that jihadists "took their revenge on its inhabitants, who happened to be mostly Yazidis who did not flee their homes".

Fear of an impending genocide against members of Iraq's Yazidi minority, whose faith is anathema to the Sunni Muslim extremists, was one reason Washington cited for air strikes it began on 8 August.

Human rights groups and residents say Isis fighters have demanded that members of religious minorities in Iraq's Nineveh province, where Kocho is located, either convert or leave, unleashing violent reprisals on any who refused.

Mohsen Tawwal, a Yazidi fighter, said he saw a large number of bodies in Kocho on Friday.

"We made it into a part of Kocho village, where residents were under siege, but we were too late," he told Agence France-Presse by telephone. "There were corpses everywhere. We only managed to get two people out alive. The rest had all been killed."

The German foreign minister, Frank-Walter Steinmeier, arrived in Iraq on Saturday to meet officials and assess what help is needed.