UN rights chief Volker Turk said he was appalled by Thursday's "horrifying strike" on two buses in Ukraine that killed 30 people.
The buses were blown up in the frontline city of Kramatorsk in one of the deadliest attacks in weeks, with bodies and debris left strewn across the road.
"I am appalled by the wave of attacks killing civilians across Ukraine over the past four days, including today's horrifying strike hitting two public buses," Volker Turk said in a statement.
"The scale and manner of these attacks -- killing and injuring people on buses, at their homes and work in different areas of the country -- is reprehensible."
He said that since Monday, at least 85 civilians had been reportedly killed and 346 others injured in Russian strikes.
Many areas of Ukraine have been plunged into darkness following a barrage of long-range strikes on electric substations and power generation plants, Turk added.
"Ukraine already faces emergency and rolling power outages, even before cold temperatures and shorter days lead to a surge in demand for electricity," he said.
The UN high commissioner for human rights also said he was deeply concerned by reports of Ukrainian attacks that have killed and wounded civilians in occupied territory and in Russia.
"Civilians are strictly protected under the laws of war and may never be attacked," he said.
"In addition, parties must take constant care to spare the civilian population," he added.
"Alleged violations must be investigated and those found responsible must be held to account. Deliberate attacks on civilians could amount to war crimes."
