Syrian government forces have shot dead scores of civilians in Aleppo, killing them as they fled or entered their homes and shooting them on the spot.
The United Nations said it had evidence militia had deliberately shot dead scores of men, women and children in eastern Aleppo, and warned the same "slaughter" could happen elsewhere. Intense bombardment from pro-government forces backed by Russia has reportedly trapped thousands more people as troops close in on the last remaining rebel-held area of the city.
The UN human rights office said that at least 82 people had been killed on the streets while fleeing or in their homes in four different neighbourhoods in the last few days.
"The crushing of Aleppo, the immeasurably terrifying toll on its people, the bloodshed, the wanton slaughter of men, women and children, the destruction – and we are nowhere near the end of this cruel conflict," UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said in a statement.
"What is happening with Aleppo could repeat itself in Douma, in Raqqa, in Idlib. We cannot let this continue," he said, referring to other rebel-held towns.
But Russia, which rejected calls for a humanitarian truce, said the atrocities were "actually being committed by terrorist groups", meaning rebel forces.
The UN human rights office spokesman told a news conference on Tuesday: "We're filled with the deepest foreboding for those who remain in this last hellish corner" of eastern Aleppo.
"Yesterday evening, we received further deeply disturbing reports that numerous bodies were lying on the streets," he added.
"The residents were unable to retrieve them due to the intense bombardment and their fear of being shot on sight."
Colville called for monitoring of the situation in order to bring perpetrators to justice.
"It needs international eyes on the situation if the fears of the worst kinds of things happening - summary executions, torture, etc - are to be allayed," he later told reporters.
Colville blamed world powers for failing to refer atrocities to the International Criminal Court for prosecution.
"The failure of the Security Council to at least involve the International Criminal Court and have some kind of judicial process has been absolutely disastrous. Because the people behind these kind of acts think they have total impunity to do what they want."
Humanitarian workers linked to Christian Aid described the "unfolding massacre" and decried "the international community's complicity" in the deaths.
Máiréad Collins, Advocacy and Programme Officer for Syria at Christian Aid, said: "What we are witnessing in Aleppo right now is horrifying. We are seeing the massacre of innocent men, women and children in their homes and on the streets, while the world watches in a state of paralysis.
"We are witnessing the unravelling of humanity before our eyes. People are posting their final goodbyes on social media, expecting that they will now be killed by regime forces. This is unbearable."
Additional reporting from Reuters.