At least 200 Iraqi families have been made to leave their homes for Mosul by Islamic State (IS) fighters and could be used as human shields, the UN warns.
A coalition of forces began an advance on the IS-held city, the second largest in Iraq, on Monday.
The UN human rights office says IS made the 200 families walk to Mosul from a nearby village on the same day.
The UN also said it was investigating reports 40 people had been shot dead by IS fighters in one village near Mosul.
The news came as IS fighters launched an attack on Kirkuk, 170km (105 miles) south-east of Mosul, killing 19 people, on Friday.
Zeid Raad Al Hussein, the UN High Commissioner for Human Rights, said there was “a grave danger that ISIL fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated,” using an acronym for IS.
As well as the 200 families made to walk from the village of Samalia to Mosul, another 350 families moved from another village, Najafia, into the city, the UN says.
The organisation said this appeared to show IS was not allowing families to flee for territory held by the Iraqi army outside Mosul, and instead was making them move into the city.
“The killings and abuses committed by ISIL fighters when they captured Mosul in 2014, and the horrors they have subjected its inhabitants to ever since, should leave us in no doubt as to the risk civilians face,” said Mr Hussein.
Civilians suspected of being disloyal to IS appeared to have been targeted already, Mr Hussein said. He did not identify the village where 40 people had been killed.
A UN spokesman said only “modest” numbers of people had so far fled Mosul as Iraqi forces and Kurdish Peshmerga fighters close in.
In Kirkuk, IS fighters attacked government buildings, killing at least six police officers, and a power station under construction, where 13 employees died.
Twelve IS fighters were also reportedly killed.
The BBC’s Richard Galpin, in northern Iraq, said IS had attempted an “audacious” counter-attack on Kirkuk that had tried to show it was still a force to be reckoned with. The attack was “clearly aimed at diverting the Iraqi army” from Mosul, he said.
A state of emergency was declared and Friday sermons were cancelled as mosques remain closed.
District police chief Brig Gen Sarhad Qadir told the BBC suicide bombers and other IS fighters had attacked three police buildings and the headquarters of a political party in Kirkuk.
The governor of Kirkuk, Najm al-Din Karim, insisted that Peshmerga fighters and counter-terrorism forces were completely in control of the situation.
He blamed the attack on IS sleeper cells.
Mosul battle: IS ‘may use civilians as human shields’}