The appeals came days after one of the worst combined missile and drone attacks on Kyiv since Russia launched its full-scale invasion of Ukraine more than four years ago.
It was the latest in a string of major attacks by Russia in recent weeks.
“The direction of the war — the escalation and the intensification that we are witnessing — risks to get out of control,” UN chief Antonio Guterres told the Security Council.
“The death spiral must stop. What is needed now is de-escalation — immediate and sustained.”
Guterres underlined “large-scale strikes launched by the Russian Federation across Ukraine on 23-24 May — and the prospect of further such attacks.”
“I strongly urge restraint. Resume negotiations and end the suffering,” UN High Commissioner for Human Rights Volker Turk said in a statement.
The UN rights office said 815 civilians had been killed and 4,174 injured in Ukraine in the first four months of 2026 — a 21-percent increase in civilian casualties over the same period last year.
“As if all these casualty figures weren’t horrifying enough on their own, following these attacks, Russian officials have publicly threatened to increase attacks across Kyiv,” Turk said.
“International humanitarian law demands that parties to a conflict take all feasible precautions to avoid civilian harm,” he said.
“These are not simply suggestions or recommendations, but binding obligations carrying legal responsibility for those involved.”
His office also pointed to an attack by Ukrainian armed forces on an educational complex in the occupied city of Starobilsk on May 21-22, in which Russian authorities say 21 people were killed and 44 injured.
“The UN Human Rights Office has conducted a thorough review of publicly-available information, which indicates that the educational facilities were operational at the time of the attack and that civilians — many of them students — were killed or injured,” it said.
Eighteen of those killed were women, it said, adding that attacks by Ukrainian armed forces had also killed and injured civilians within Russia itself.
Turk called on both Russian and Ukrainian authorities to conduct “prompt, independent, and effective investigations and hold those responsible accountable”.