Iraqi Shiite militia fighters hold the Islamic State flag as they celebrate after breaking the siege of Amerli by Islamic State militants. (Youssef Boudlal/Reuters)

A day after breaking through a siege by Islamic State militants on the Shiite town of Amerli, Iraqi forces and pro-government militias on Monday pushed their way into nearby villages they accuse of helping to enforce the months-long blockade.

The arrival of the Shiite-dominated armed groups in neighboring Sunni towns raised fears that Sunnis could be targeted in revenge killings.

Sectarian bloodshed has been on the rise since Islamic State militants rampaged across northern Iraq in June, targeting Shiites and minority groups and reinvigorating the country’s violent Shiite militias.

The Sunni jihadist group had surrounded Amerli, a poor Shiite farming hamlet, and had cut off access to food, water, and electricity for two months.

The breakthrough on Sunday came as fighters from Iraq’s various military and paramilitary forces fought their way into the town, with the help of U.S. airstrikes on Islamic State targets in the area.

On Monday, the fighting continued as Iraqi, Shiite and Kurdish Pesh Merga forces swept into the nearby Sunni town of Suleiman Beg and other Sunni villages.

“Suleiman Beg and other villages around Amerli have been fully liberated,” said Na’im al-Aboudi, the spokesman for Asa’ib Ahl al-Haq , an Iranian-trained Shiite militia that once fought U.S. forces and played an active role in some of Iraq’s worst sectarian bloodshed.

By Monday evening, the militia, along with Iraqi troops, were in control of the town and a strategic road linking it to Amerli, Aboudi said.

Much of Suleiman Beg’s population had fled by the time Iraqi forces arrived, area residents and militia fighters said.

Those who remained were pleased to be rid of the Islamic State and its “tactics,” said Shalal Abdul, a Sunni and the head of the local council in the nearby town of Tuz Khurmatu.

But tensions also ran high as area Sunnis who had initially welcomed Islamic State’s presence because of its opposition to Iraq’s Shiite-dominated government, worried that they would now be accused of collaborating with the militants, Abdul said.

“The residents are very concerned,” he said. “They are calling us to ask us to please explain that they were not collaborators and were just forced to stay.”

Meanwhile, the United Nations said Monday it was able to deliver 45 metric tons of humanitarian aid to Amerli, where thousands of Shiites from Iraq’s Turkmen minority had been trapped.

Iraqi television networks broadcast footage of civilians and fighters cheering in celebration as the aid trucks and prominent visitors rolled in.

The U.N. also said that more than 1,400 Iraqis were killed in acts of violence across the country in August.

Iraq’s outgoing Prime Minister Nouri al-Maliki traveled Monday by helicopter to Amerli, where he addressed a crowd of locals. Maliki’s transport minister, Hadi al-Ameri, who heads a powerful Shiite militia, stood at his side.

Amerli, Maliki said, was a “second Karbala” — the scene of the 7th century battle between Muslim armies that came to define the split between Sunnis and Shiites.

“All of Iraq will be a cemetery for daaish,” he said, using an acronym for the Islamic State.

The London-based watchdog Amnesty International on Tuesday accused the Islamic State of ethnic cleansing in areas of northern Iraq under the group’s control.

In a 26-page report titled “Ethnic Cleansing on a Historic Scale,” Amnesty said the jihadists have systematically targeted non-Arab and non-Sunni Muslim communities since sweeping across northern Iraq this summer.

Islamic State militants have abducted, killed and expelled scores of people from minority communities in a deliberate policy of ethnic cleansing and that likely amounts to war crimes, Amnesty said. The rights group documented three major incidents in August in which jihadists shot and killed scores of men and boys belonging to Iraq’s Yezidi religious minority, after separating the victims from the rest of the villagers and transporting them to remote areas for execution.

Communities vulnerable to the Islamic State onslaught “risk being wiped off the map,” the report said. Hundreds of women and children have been kidnapped by the militants, and scores more men are also feared abducted.

Cunningham reported from Irbil. Mustafa Salim in Baghdad contributed to this report.