Iraqi security officials have reported that the bodies of 150 members of an Iraqi Sunni tribe fighting the Islamic State (Isis) alongside the military have been found in a mass grave in Iraq's restive Anbar province.
The development came one day after Islamic State reportedly lined up and shot dead 30 Sunni men in the same area.
Anbar provincial chairman Sabah Karhout said the Sunni tribesmen, allied with the government and members of the security forces, were captured when jihadists conquered the town of Hit, which is 140km (85 miles) west of the capital, Baghdad.
Earlier in October, Iraqi troops helped by Sunni fighters from the Albu Nimir tribe were forced out from the town, Anbar's fifth-largest city, after heavy clashes with IS.
During its advance, in September IS seized the Iraqi military base of Camp Saqlawiyah, 45 miles (70km) west of Baghdad, amid claims the government failed to provide adequate support to its troops stationed there.
The Obama administration and Iraq have teamed up with some Sunni tribesmen to fight against IS in the hope of rekindling the same Sunni uprising that shattered an al-Qaida-linked group at the height of Iraq's sectarian civil strife between 2005 and 2007.
At the time, the US military recruited and paid Sunni tribes to lead the struggle against the jihadists. But the Shi'ite-dominated government of former prime minister Nouri al-Maliki fuelled sectarianism and disenfranchised the Sunni community from power, driving them to support IS in the first instance.
The US believe Iraqi Prime Minister Haider al-Abadi's newly formed government can be more inclusive of Sunni Muslims and Kurds, helping the American-led coalition to stop IS.