Sunni insurgents in northern Iraq have carried out summary executions of hundreds of captured soldiers, the UN’s human rights chief says.
The systematic killings “almost certainly amounted to war crimes,” according to Navi Pillay.
Her remarks came as the militants, led by the Islamic State in Iraq and the Levant (ISIS), were reported to have seized the strategic city of Tal Afar.
The rebels occupied several key cities last week, but some towns were retaken.
Security in Baghdad has been stepped up after the insurgents threatened to march on the capital.
Reports are emerging that an army helicopter has been shot down in fierce clashes near the city of Fallujah, located just 70km (45 miles) west of Baghdad.
President Barack Obama says the United States is deploying up to 275 military personnel to Iraq to protect American interests there.
One reason Tal Afar was important for the government to hold was that the city was the state’s only outpost in the entire province of Nineveh, which fell to the Sunni militants last week as the army collapsed.
It is also strategically significant, straddling the main highway from Mosul, the provincial capital, to the Syrian border.
However, assuming Tal Afar has indeed fallen to the militants, it does not mean they have a direct link to Syria – the border crossing at Rabia is controlled on the eastern side by Iraqi Kurdish Peshmerga fighters, and on the western side by the Popular Protection Units (YPG), the armed wing of the Syrian Kurdish Democratic Union Party (PYD), which is fiercely hostile to ISIS.
Tal Afar is important for other reasons too. Most of the areas through which the Sunni militants swept last week were largely Sunni populated. Tal Afar has a big Shia community, from the Turkmen minority, perhaps one reason why it held out longer than any other town in Nineveh.
Some observers believe it is important for Iran – which sees itself as the custodian of the Shia – that Tal Afar should not be allowed to fall, and that they would sooner or later wreak revenge, especially if abuses were committed during or after its capture.
Ms Pillay said the UN had evidence from its own human rights monitors in Iraq that non-combatants had been summarily executed over the past five days, including members of the security forces who had surrendered or been captured.
Her comments came after Sunni militants posted photos online appearing to show fighters massacring Iraqi soldiers.
In a video circulated online, ISIS fighters were also seen taunting captured troops with threats of decapitation.
The Iraqi military earlier said the pictures were real, but their authenticity has not been independently confirmed. The US condemned them as “horrifying”.
The insurgents’ lightning advance began last week with their capture of the cities of Mosul and Tikrit.
Fighting in Tal Afar began on Sunday, with mortar shelling of some districts as the militants tried to enter the city in Nineveh province.
Tal Afar, which has a mixed Sunni and Shia population, lies between Mosul and the Syrian border.
The city of 200,000 people was taken just before dawn on Monday, Mayor Abdulal Abdoul told the Associated Press news agency.
Eyewitnesses said militants in pick-up trucks mounted with machine guns and flying black jihadist banners were roaming the streets as gunfire rang out.
But the government insists it still largely controls the town and that ISIS fighters would be swiftly dealt with, says the BBC’s Jim Muir in northern Iraq.
The government also announced it had “regained the initiative” against an offensive by Sunni rebels.
The defence ministry released footage on Monday which it said was of air strikes on ISIS targets north of Baghdad, including Mosul and Samarra provinces.
Government forces were also reported to be building up in the city of Samarra, north of Baghdad, for a counter-offensive on Tikrit.
The US earlier announced it might use drone strikes to halt the ISIS advance.
The Pentagon said US officials were also open to holding direct talks with Iran over Iraq, but there was “no plan to co-ordinate military activity” between the two countries.
Iranian President Hassan Rouhani said he would consider co-operation if the US took action.
The USS George HW Bush aircraft carrier has already been deployed to the Gulf, accompanied by two more warships. But Washington says no US troops will be deployed on the ground.
Britain reiterated on Monday that it had no plans for military intervention in Iraq.
The US has begun evacuating some diplomatic staff from Baghdad, moving them to Kurdish-controlled territory in the north-east and to Basra in the south.
The UN confirmed it had also moved 58 of its personnel from Baghdad to Amman in neighbouring Jordan, adding that more relocations were possible.
An estimated 500,000 people have been displaced internally as a result of recent fighting in Mosul.