ISIS militants killed more than 300 members of a Sunni tribe in a recent series of executions, the Iraqi government said Monday.

Some of the 322 people executed were women and children, Iraq's Ministry of Human Rights said. The dead belonged to the Albu Nimr tribe, known for its fighting skill.

Pentagon spokesman Rear Adm. John Kirby said Washington hasn't been able to confirm those figures, but "we have no reason to doubt their authenticity."

The latest incident came Saturday when 75 members of Albu Nimr were taken from their homes and killed in the desert near the town of Hit, said one of the tribe's leaders, Sheikh Nabil Al-Ga'oud.

It has been unusual for ISIS militants, who refer to themselves as the Islamic State, to kill women and children.

"We are not surprised by their actions," Al-Ga'oud told CNN. "Their religious leaders have given them an order to kill all of our tribe and take everything that we own because we are fighting against ISIS."

He said nine children and six women were killed in Saturday's attack.

A spokeswoman for the U.S. State Department condemned the ISIS attacks.

"This proves, once again, that (ISIS) does not represent anything but its warped ideology and provides more evidence, if any were needed, why our coalition partners, including Iraqis from every background, must work together to defeat these terrorists," Jen Psaki said at the department's daily briefing.

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

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 of out weapons and supplies.

Al-Ga'oud said an attack is "imminent," adding that "we are getting the support we need," in an apparent reference to backing by Iraq's central government.

A 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.

Kirby said U.S. advisers were working hard to improve the competence and capability of Iraq's army.

"The Iraqis are pushing back. They are going on an offensive against (ISIS) throughout the country," he said. "Now it is not a major offensive, but they are reaching out to some areas."

(CNN)