The UN rights office said its staff had viewed two separate pieces of video footage "showing what appear to be summary executions that Ahrar al-Sharqiya armed group carried out, which is affiliated with Turkey, on 12 October."
Spokesman Rupert Colville said the footage showed the fighters filming themselves capturing and executing three Kurdish captives" on a main highway.
"Only one of the captives appeared to be wearing military uniform,” adding the office had also received reports of Khalaf's execution the same day "on the same highway".
He said the UN was working to verify the footage and confirm the details of the events, but stressed that under international law, "summary executions are serious violations, and may amount to a war crime."
"Turkey could be deemed responsible as a state for violations committed by their affiliated armed groups, as long as Turkey exercises effective control over these groups, or the operations in the course of which those violations occurred."
Colville called on Ankara "to immediately launch an impartial, transparent and independent investigation into both incidents."
Colville said that the UN rights office had been able to verify a number of civilian deaths each day in northeastern Syria due to "airstrikes, ground-based strikes and sniper fire."
He pointed in particular to an airstrike that hit a convoy of vehicles on Sunday, which reportedly killed "at least four civilians, including two journalists" and injured dozens of others.
He said the UN was "appalled" by a number of attacks on medical facilities in the region.