Sheikh Naeem al-Gaoud told the BBC that members of the Al Bu Nimr tribe were shot dead in the village of Khanizir, in Anbar province, on Sunday night.

He said the tribesmen were killed because they had relatives serving in the Iraqi security forces.

The Al Bu Nimr have played an important role in battling terrorists for years.

In 2014, ISIS militants killed more than 500 members of the tribe, but its leaders vowed to fight on.

The tribe, which is believed to include more than half a million people, confronted ISIS after it seized control of swathes of western and northern Iraq that June.

Members of the tribe also helped the US military counter al-Qaeda in Iraq, a precursor to ISIS, during the so-called "Sunni Awakening" in 2006 and 2007.

Sheikh Gaoud said the men targeted on Sunday had fled to Khanizir with their families from the homes in the al-Furat district of Ramadi after ISIS captured Anbar's provincial capital last May.

in October 31, 2014 ISIS militants executed at least 220 Iraqis in retaliation against tribe's opposition to their takeover of territory west of Baghdad.