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A small group of U.S. special forces have been on Iraq’s Mount Sinjar carrying out what U.S. officials told CNN on Wednesday was an assessment of the humanitarian crisis where thousands of minority Yazidis are trapped by extremist fighters.

The group spent 24 hours on the mountain trying to determine just how many Yazidis are there and the conditions they face, said the officials, who spoke on condition of anonymity.

The revelation follows news the United States deployed 130 military advisers to get a firsthand look at the humanitarian crisis unfolding as ISIS fighters threaten Iraq’s ethnic and religious minorities: Yazidis, Christians and Kurds.

Nowhere is the crisis more evident than Mount Sinjar, where as many as 40,000 minority Yazidis are hiding after fleeing the advance of fighters with the Islamic State, formerly known as the Islamic State of Iraq and Syria, on the town of Sinjar.

CNN first learned about the U.S. military operation on Tuesday but withheld the information at the request of U.S. officials out of concern for the safety of the troops. CNN did not report about the operation until the troops left the mountain.

News of the American troops came the same day a senior ISIS commander told CNN that ISIS fighters abducted more than 100 Yazidi women and children from Sinjar.

The ISIS commander, who has knowledge of the events that unfolded, said the fighters killed a large number of men when they took over the town more than a week ago.

“At that time, they took Yazidi women and children, and I can confirm those women and children have entered Mosul,” the commander said by telephone. “…The Islamic State is taking this opportunity to call them to Islam.”

While CNN cannot independently confirm the claim, it follows previous reports by survivors who describe ISIS fighters grabbing families and separating the men from the women and children.

The plight of the Yazidis, coupled with the ISIS assault against Iraq’s semiautonomous Kurdish region, prompted the United States to begin targeted airstrikes over the past several days. The aim, according to U.S. President Barack Obama, is to help protect U.S. personnel in the area and to destroy ISIS positions around the mountains to ease the threat to minority groups.

President Barack Obama this week ordered nearly 130 advisers to the Kurdish capital of Irbil to assess the humanitarian crisis, and U.S. officials say it’s likely that a small number of them will travel to the mountain area to get a first-hand look at what might be possible.

The advisers, made up of Marines and special operations forces, join hundreds of other American advisers already in the country advising Iraqi troops in their fight against ISIS, an offshoot of al Qaeda in Iraq.

The group has waged a brutal campaign while seizing large areas of territory this year in Iraq, aiming to establish a caliphate.