Zeid Ra'ad Al Hussein was referring to an incident where 10 people were wounded in the town of Cizre on January 20. The group of people, which included two opposition politicians, came under gunfire while trying to rescue and assist others caught up in earlier clashes.
Zeid said he had seen the "extremely shocking" footage, which appears to show a man and a woman waving white flags and pushing a cart — thought to possibly contain bodies — across a street.
"As they reach the other side, they are apparently cut down in a hail of gunfire," Zeid said.
The UN human rights chief also raised concerns that the cameraman, who was also wounded in the incident, may face arrest and charges under a recent "clampdown on media" in Turkey.
Fears Over Civilian Safety
The incident is the latest in long list of recent condemnation of Turkish security forces, who are engaged in clashes with Kurdish rebels in some parts of Turkey's Kurdish majority southeast.
There are grave fears for many residents caught up in the fighting between government forces and rebel militia groups, with reports that as many as six civilians have been killed, while many others are being denied access to basic supplies and medical attention.
Turkey's Peoples' Democratic Party (HDP) said that 31 people, including some in a critical condition, had been trapped inside the basement of a building in Cizre, while many ambulances have been denied access to some areas.
Amnesty International's Turkey Researcher Andrew Gardner has accused the Turkish government of failing to respond to the desperate pleas of those injured and trapped.
"This is a desperate situation: injured individuals, some of whom are apparently bleeding heavily, are at grave risk of dying if they do not urgently receive medical care.
"Lives are dangling in the balance and it defies belief that the authorities cannot find a solution. Whatever the circumstances which led to these men's injuries, they have a right to receive life-saving medical treatment and the state should facilitate, not impede, such care."
Turkish authorities have imposed 24-hour curfews in some Kurdish-majority areas in the country's southeast, after violence between Ankara and those affiliated to the Kurdistan Workers' Party (PKK) reignited in July 2015.
More than 150 civilians are reported to have been killed in the clashes, including women, children and the elderly.