Displaced Iraqis, who fled regions controlled by the Islamic State near Fallujah, carry their belongings Feb. 9 as they arrive on the eastern outskirts of Ramadi, after pro-government troops retook it from jihadists. (Moadh Al-Dulaimi/AFP/Getty Images)

Sunni tribesmen have attacked Islamic State militants in the Iraqi city of Fallujah, the Iraqi military and local officials said Friday, overrunning and burning one of their headquarters as resentment against the group grows.

After the unrest in two neighborhoods, Issa al-Issawi, the mayor in exile of the city 45 miles west of Baghdad, warned that there would be “mass slaughter” unless the tribesmen received assistance.

The flare-up was the first notable sign of armed opposition to the Islamic State inside the city since the group took control of Fallujah more than two years ago. Ill will against the group has been mounting in recent months amid what the United Nations has described as a growing humanitarian crisis inside the city.

Iraqi forces have advanced farther west, retaking Anbar’s provincial capital of Ramadi and leaving Fallujah isolated and surrounded.

The military siege has prevented food and medical supplies from entering, while the Islamic State has not allowed civilians to leave.

[Trapped by the Islamic State, Iraqis in Fallujah say they are going hungry, too]

“The people are so frustrated because of the siege,” said Rageh Barakat, a member of Anbar provincial council’s security committee. “The situation is dire. There is no food, and they know the [Islamic State] is hoarding it. They’ve had enough.”

Barakat said Friday that the Islamic State was carrying out widespread arrests in the city as a result of the violence.

There were varying reports of how the clashes began; with mobile phone networks cut in the city, communication is difficult.

Barakat, who said he has been contacting groups in the city that oppose the Islamic State’s rule for the past four months, said clashes began in the northern Jolan neighborhood Thursday. Militants from the Islamic State’s “hisbah” — who enforce the group’s moral code on the street — “humiliated” two elderly men when the men complained about a lack of food, he said. Other security officials said it stemmed from a dispute with a butcher.

The tribesmen “attacked the hisbah headquarters, burned it, and they were able to kill those inside,” Issawi said, adding that 10 were slain.

Barakat also said that the headquarters was burned and overrun. He put the death toll at four. It was not immediately possible to verify the reports.

Neither Issawi nor Barakat had a figure for deaths of Sunni tribesmen. Clashes spread to the Nazzal neighborhood Friday, they both said, disputing reports that the tribesmen had managed to hold any ground.

“It’s impossible to control any area,” Issawi said. “But there’s huge tension right now.”

The support of Iraq’s Sunni population is seen as key in the fight against the Islamic State. Issawi urged the government to take action to build on the momentum inside the city, which could “fall very quickly,” he said.

Some military commanders are arguing that Iraq’s focus should be on the province of Anbar rather than Mosul, the Islamic State’s stronghold in northern Iraq. The Iraqi military has drawn up plans to attack the Anbar city of Hit, in the west of the province.

[With Islamic State under pressure, Iraqi forces hope to reclaim city of Hit]

“There is a volcano of resentment boiling inside Fallujah,” the Iraqi military said in a statement detailing the clashes. Civilians are waiting for the security forces to enter to “carry out a revolution,” it said.

The unrest was led by the Juraisat tribe, with the Halabsa and Mohamda tribes joining in, the military said.

However, Barakat said the fighting did not involve traditional tribal structures, but rather groups of disaffected young men.

“It’s not the tribes fighting,” he said. “It’s the young people rising up.”

He said that when they first began to contact tribesmen four months ago, only a small number of dissenters were willing to organize. But that number has burgeoned as the humanitarian situation in the city has worsened, he said.

“We tried to deliver them some weapons a month ago, but it wasn’t possible,” Barakat said.

Issawi said outside intervention is urgently needed, before those who are leading the fight against the militants are killed.

“If those groups inside aren’t supported, Daesh will have huge revenge,” he said, referring to the Islamic State by its Arabic acronym. “There will be the biggest bloodshed ever.”

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