Story highlights Hundreds of members of Albu Nimr tribe have been killed by ISIS

ISIS fighters carried out public executions of tribesmen in Hit

Group had more than 50 members abducted by ISIS Saturday

Earlier in the week, many others had been killed

They were taken from their homes, some pulled from their beds, in the middle of the night.

They were fathers, brothers and sons, members of the U.S.-allied Albu Nimr tribe -- the Sunni clan considered among the last holdouts against ISIS in Iraq's western desert.

About 50 members of the tribe were abducted in Hit in Anbar province, west of Baghdad, during the early morning hours on Saturday, Sheikh Nabil Al-Ga'oud, a tribal leader, told CNN.

Their fate is unknown. But Al-Ga'oud and others believe they are likely dead, the latest casualties of ISIS who have killed hundreds of members of the tribe in mass executions in recent days.

The Albu Nimr, who number in the tens of thousands, are ready to fight to take back Hit, Al-Ga'oud said. The city was seized last month by ISIS fighters after weeks of fighting the tribesmen.

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Hit and neighboring Ramadi were holdouts in the Sunni-dominated Anbar province after ISIS swept in from Syria, taking town after town in the western province. Albu Nimr tribesmen were among those who fought them until they began running out of weapons and supplies.

Tribal leader says fighters ready to strike ISIS

Albu Nimr has a strong force ready to attack Hit, but they need to get the final OKs from the government and also try to coordinate with the Americans, Al-Ga'oud said.

Anbar was the scene of a bloody insurgency during the U.S.-led war in Iraq until an uprising by Sunni tribes, including Albu Nimr, in 2006 took on al Qaeda in Iraq -- the forerunner to ISIS, also known as ISIL.

U.S. officials maintain that Iraqi support for Sunni tribes going on the offensive against ISIS will be a necessary part in the effort to defeat the militants, who refer to themselves as the Islamic State.

In a news conference this week, Gen. Martin Dempsey, the Joint Chiefs of Staff chairman, signaled the possibility of the new role when he said the Iraqi government had not yet requested U.S. military support in efforts to defend a Sunni tribe that has suffered mass executions at the hands of ISIS militants.

"That's why we need to expand the train-advise-and-assist mission into the ... Anbar Province," Dempsey said. "But the precondition for that is that the government of Iraq is willing to arm the tribes."

Dempsey said the United States had "positive indications" the Iraqi government was prepared to do that but had not yet acted.

There is no indication from Pentagon officials on when such a mission could begin.

Public executions, mass graves

The abduction and suspected killings follow reports this week of public executions and the discovery of mass graves containing the bodies of tribesmen killed by ISIS.

The bodies of an estimated 200 members of Albu Nimr were found in a mass grave just outside Hit, a senior Iraqi security official told CNN. The tribesmen were captured by ISIS fighters after it took control of the area, the official said.

Another 48 tribesmen were marched through the streets of Hit before they were publicly executed, the official said.

And a mass grave was found in nearby Ramadi, according to Iraqi media accounts. Video of those executions had been uploaded to the Internet.

"We are deeply concerned by reports of mass executions of Sunni tribesmen in Anbar province by ISIL and strongly condemn the brutal actions that ISIL continues to perpetrate against the Iraqi people," U.S. State Department spokeswoman Jen Psaki said.

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"...ISIL's indiscriminate crimes prove, yet again, that it is targeting all Iraqis, regardless of faith or region."

Albu Nimr fights back

Albu Nimr is known for its fighting skill and resistance.

2003 Brookings Institution report observed that, though most Iraqi Sunni tribes were loyal to Saddam Hussein in the days when he ran the country, the Albu Nimr tribe had mounted a protest against the former Iraqi strongman in 1995 after the execution of a noted member. The protest was put down by paramilitary forces loyal to Saddam.

In general, however, Saddam respected the Albu Nimr.

Since Saddam's fall, they've been tapped to oppose al Qaeda in Iraq. They were also part of last year's Sunni uprising against the former Prime Minister Nuri al-Maliki and his Shiite-dominated government.

They have been fighting ISIS but say they haven't received much support from the Iraqi government and international coalition members.