Islamic State appears to be using tens of thousands of people as human shields in and around Mosul, where Iraqi forces are waging a large-scale offensive aimed at retaking the country's second largest city, the UN human rights office has said.
The High Commissioner for Human Rights has also received reports of more than 200 people being killed for refusing to comply with IS orders or previously belonging to Iraqi security forces.
It said "credible reports" suggest IS has been forcing tens of thousands into Mosul from their homes in districts around the city.
Spokeswoman Ravina Shamdasani said in Geneva: "Isil's depraved, cowardly strategy is to attempt to use the presence of civilian hostages to render certain points, areas or military forces immune from military operations, effectively using tens of thousands of women, men and children as human shields."
IS militants were reportedly going door to door in villages south of Mosul earlier this week, ordering hundreds of civilians at gunpoint on a forced march north into the city, "to be able to keep them close to military installations, close to their offices" she added.
Ms Shamdasani said: "Many of those who refused to comply were shot on the spot, and even among those who did comply, many of them - including 190 former ISF (Iraqi security forces) officers and 42 other civilians - were shot dead."
The officers, who had been brought from a district south of the city, were killed at a military base in Mosul while the civilians were shot in the Hamam al-Alil district for refusing to join IS.
Those killings reportedly happened on Wednesday, and another 24 former officers with the ISF were reportedly killed on Tuesday.
Ms Shamdasani said the reports "have been corroborated to the extent possible".
Iraq launched a massive operation on October 17 aimed at retaking Mosul, which fell to IS in a matter of days in summer 2014. Iraqi forces are advancing from several directions, but are still well outside the city itself.
The UN and rights groups have expressed fears that more than 200,000 civilians could be displaced in the opening weeks of the offensive. Mosul is still home to more than a million people.
IS has built up elaborate defences on the outskirts of the city, including an extensive tunnel network, and has planted large numbers of explosive booby traps to slow the troops' progress.
The US military said coalition air strikes had targeted a group of around 50 vehicles IS fighters were gathering to transport Iraqi civilians into Mosul from the city's outskirts.
Colonel John Dorrian, a Baghdad-based military spokesman, said it was believed the militants intended to use the civilians as human shields.
He said 40 to 45 of the vehicles were hit in the US attack before any civilians were put in them.
The US military also said Iraqi forces have retaken 40 villages from IS near Mosul since the operation began, but most of the fighting has taken place in a belt of sparsely populated farming communities ringing the city.
US Air Force Brigadier General Matthew C Isler said Iraqi troops were consolidating gains made east and south of the city earlier this week, but insisted "momentum" was still on their side.
He said the US-led coalition has stepped up air strikes against the militants, carrying out three times as many as it did during previous campaigns to drive IS from other Iraqi cities.