BAGHDAD/BARTELLA, IRAQ ― U.S.-backed Iraqi forces moved closer on Wednesday to a town south of Mosul where aid groups and regional officials say Islamic State has executed dozens of prisoners.

A military statement said security forces advanced to the edge of Hammam al-Alil, a thermal water resort, after an elite unit breached the eastern limits of Mosul, the ultra-hardline group’s last major city stronghold in Iraq.

The battle that started on Oct. 17 with air and ground support from a U.S.-led coalition is shaping up as the largest in Iraq since the U.S.-led invasion of 2003.

Mosul still has a population of 1.5 million people, much more than any of the other cities captured by IS two years ago in Iraq and neighboring Syria.

The United Nations cited reports on Tuesday that Islamic State, which is also known as ISIL, is attempting to displace Hammam al-Alil’s estimated population of 25,000 for use as human shields and protection against air and artillery strikes.

“We have grave concerns for the safety of these and the tens of thousands of other civilians who have reportedly been forcibly relocated by ISIL in the past two weeks,” U.N. human rights spokeswoman Ravina Shamdasani said.

The town, 15 km (9 miles) south of Mosul, had a pre-war population of 65,000, a local official said.

Aid organizations, local officials and Mosul residents have cited reports that IS has executed dozens of people in Hammam al-Alil and barracks nearby on suspicion of planning rebellions in and around Mosul to aid the advancing troops.

Abdul Rahman al-Waggaa, a member of the Nineveh provincial council, told Reuters last week that most of the victims were former police and army members. The men were shot dead, he said, quoting the testimony of remaining residents of the villages and people displaced from the area.

FEARS OF A HUMANITARIAN CRISIS

The U.N has said the Mosul offensive could trigger a humanitarian crisis and a possible refugee exodus if the civilians inside in Mosul seek to escape, with up to 1 million people fleeing in a worst-case scenario.

The International Organisation for Migration said nearly 21,000 people have been displaced since the start of the campaign, excluding thousands of villagers taken into Mosul by retreating jihadists who used them as human shields.

Elite Counter Terrorism Service (CTS) troops were the first

to breach Mosul’s official boundary this week. They said on Tuesday they were in control of the state television station.

A CTS commander, Lt. General Abdul Ghani al-Assadi told reporters in Bartella, a village west of Mosul, that the unit will pause its advance on the eastern front because of bad weather.

“God willing the next stage will begin within hours. This depends on the weather,” he said.

A curfew had been imposed on the recaptured eastern suburb of Kokjali, he said, to protect residents from mortar bombs fired by the insurgents.

Kurdish Peshmerga fighters are also deployed on the eastern and northern fronts, and Iranian-backed Iraqi Shi’ite militias are attacking Islamic State west of Mosul.

The involvement of pro-Iranian militias is causing alarm in Turkey which has had troops deployed north of the city since last year to train and support Sunni Arab volunteers who also want to take part in the battle.

Turkey’s army has begun deploying tanks and other armored vehicles to the town of Silopi near the Iraqi border. Turkish Defence Minister Fikri Isik said on Tuesday the move was related to the fight against terrorism and developments across the border.

Turkey says it has a responsibility to protect ethnic Turkmens and Sunni Arabs in the area around Mosul, once part of the Ottoman Empire. It fears both PKK militants and Shi’ite militias, which the Iraqi army has relied on in the past, will be used in the campaign and stoke ethnic bloodletting.

Iraqi Prime Minister Haider al-Abadi told a news conference after a cabinet session in Baghdad on Tuesday that tensions with Turkey have eased in the last week, but warned that Iraq would respond to any “violation” of its territory.