Iraqi special forces have moved more than 1,000 people from villages near the front lines of the battle to retake the Islamic State-held city of Mosul and surrounding areas, where the U.N. says militants have committed a number of atrocities in recent days, officials said Wednesday.

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Special forces Maj. Gen. Haider Fadhil said residents of Tob Zawa and other villages were taken to a camp in the nearby Khazer region for their safety. The International Organization for Migration says around 9,000 people have been displaced since the operation to retake Mosul began on Oct. 17.

The special forces were undertaking cleanup operations in areas retaken from the militants to the east of the city, where troops uncovered a vast tunnel network used by IS to shuttle fighters and supplies by motorcycle, Maj. Salam al-Obeidi said.

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Iraqi forces have been pushing toward Mosul from several directions since the launch of the wide-scale offensive, which involves more than 25,000 Iraqi soldiers, Kurdish forces, Sunni tribal fighters and state-sanctioned Shiite militiamen. It is expected to take week, if not months, to drive IS from its last urban bastion in the country.

The militants have had months to prepare for the long-awaited operation and are believed to have developed extensive defenses in and around the city.

"They've really dug in, literally, and started putting up the berms, the trenches, the tunneling systems," said a U.S. military official, speaking on condition of anonymity to discuss the subject with reporters. "And they're quite extensive tunneling systems, some of them stretching upward of two kilometers (over a mile)."

He said Iraqi forces have found homes near Mosul where the lights are wired with explosives that detonate if you flip the switch. Inside Mosul, the IS group has set up large concrete barriers known as T-walls, blocking off several streets.

The Islamic State group is also believed to have grown increasingly brutal as it seeks to eliminate any potential threats from among the local population, killing alleged spies as well as former members of the Iraqi security forces.

The U.N. High Commissioner for Human Rights said Tuesday that IS appears to have carried out a number of atrocities in recent days in and around Mosul, including killing 50 former Iraqi police officers they had been holding in a building near the city.

Spokesman Rupert Colville said Iraqi forces found the bodies of 70 civilians who had been shot dead in the Tuloul Nasser village, some 20 miles south of Mosul. He said it was not immediately clear who was responsible for the killings, and cautioned that it was hard to immediately verify the reports.

He told reporters in Geneva that the U.N. rights body also had reports that the militants gunned down 15 villagers south of the city and threw their bodies in a river. In the same village, IS tied six people to vehicles by their hands and dragged them around because they were related to a tribal leader battling the extremists, he said.

"We very much fear that these will not be the last such reports we receive of such barbaric acts," Colville said.

The U.N. and rights groups have expressed fears that IS may use civilians as human shields as Iraqi forces converge on the country's second largest city, which is still home to more than a million people.

Colville said IS fighters shot dead three women and three girls because they were lagging behind as the militants were forcibly relocating them to another district south of Mosul. He said they were lagging behind because one of the girls had a disability.

Colville also expressed concern over the "severe measures" taken by local Iraqi Kurdish authorities in Kirkuk following a massive IS assault on the northern city last week. He said they had ordered all internally displaced people living outside established refugee camps to vacate their residences and move into the camps, some of which are already full.

"We understand that hundreds of families have now been evicted by Kurdish Security Forces, and are worried that if the evictions continue, it could significantly complicate the already alarming situation of mass displacement in the region," he said.