ISIS militants in the Iraqi city of Mosul have reportedly executed at least 232 people and forced thousands of others to relocate at gunpoint in order to transform them into human shields, the United Nation’s human rights office said on Friday.

The apparent executions and forced movements suggest the jihadist group is turning to increasingly brutal measures as it attempts to defend Mosul from a large-scale military offensive launched on Oct. 17 meant to restore government control over the city, which was once Iraq’s second largest.

The U.N.’s Office of the High Commissioner for Human Rights said that Islamic State operatives forced thousands of families from four separate sub-districts of Mosul to move to ISIS “strongholds.” As a result, population of the town of Hammam al-Alil, south of Mosul recently swelled from 23,000 to 60,000. The document cited “credible reports” received by the U.N.

Read More: Iraqi Forces Find a Bomb Factory and Tunnels On the Long Road to Mosul

According to the U.N., the 232 people executed include 190 former members of the Iraqi security forces, who were killed last Wednesday at al-Ghazlani military base in Mosul. Reports indicate that another 42 civilians were shot in the head at al-Izza military base after they refused to join ISIS, the statement adds.

Supported by a U.S.-led international coalition, the Iraqi military and Kurdish militias mobilized tens of thousands of troops last week in an attempt to dislodge ISIS from Mosul, the largest city under the extremist group’s control. The jihadists have held the city since they seized it in a lightning sweep across Iraq in June 2014. The battle signals the turning of the tide in the international war against the group, following months in which ISIS has lost control of major cities in Iraq and key sections of its territory in Syria. But ISIS is currently mounting a ferocious defense of Mosul including the use of snipers, improvised bombs, and dozens of suicide bombers. Weeks or possibly months of hard fighting lie ahead before the city is retaken.

Read More: Iraq Takes on ISIS as the Battle for Mosul Begins

The U.N. statement issued on Friday is consistent with the testimony of Iraqi civilians who recently fled ISIS-held areas of northern Iraq interviewed by TIME in recent days. Their accounts suggest that in recent weeks, ISIS has tightened its system of social control in anticipation of the attack on Mosul, and also increased the number of executions of suspected smugglers, former members of the Iraqi security forces and others who ran afoul of the organization. “They’re putting the bodies on stockades so people walking in street can see them. There’s a bad smell in the city,” said Mariam Mohamed Hamad, 21, who recently fled the ISIS-controlled city of Hawija, in an interview in the Debaga displacement camp outside the town of Makhmour. She said the bodies were executed smugglers, as well as members of the Iraqi army killed in battle.

Hawija, a city with an estimated population of 115,000, is now cut off from Mosul, the much larger city to the north, following an Iraqi army offensive that severed the north-south highway. As a result, residents who recently fled say the price of food has soared. A bottle of oil can cost as much as 100,000 Iraqi Dinars, or about $85. A kilo of rice cost up to $36 according to Hamad. In addition ISIS has tightened its ultraconservative restrictions on every day life. “If they see anyone smoking they’ll punish them immediately, maybe kill them. The system is tough,” she said.

Read More: All Quiet on the Mosul Front as Iraqi and Kurdish Forces Hold Positions

Several former residents of ISIS-controlled areas said that increasingly severe treatment of civilians is eroding the degree of popular support that existed for the group. “In the beginning people liked them. They treated people in a good way, but the punishments are harsh,” says Omar Walid Mohamed, 20, from the town of Qayyarah, south of Mosul, which the Iraqi military retook last August.

Within the city of Mosul, some residents are also now actively resisting ISIS in anticipation of the coming battle, according to multiple reports and residents who fled the city. “They don’t show up, they just kill ISIS fighters and they hide themselves. No one knows who they are,” said a 60-year-old man from Mosul who fled with his family about two months ago. In an interview in a displacement camp near Qayyarah, he asked to have his name withheld for fear of retaliation from ISIS.

ISIS officials have also intensified measures to cut civilians off from the outside world, confiscating satellite dishes and other equipment. “ISIL has seized many SIM cards, and those who successfully managed to hide them, fear to use them,” according to an anonymous blogger who writes under the name Mosul Eye and claims to be reporting from inside Mosul. “Mosul has entered the atmosphere of the war. The bombardment is continuous on many areas of the city,” the writer also said.

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