UN Secretary General António Guterres has said he is "gravely concerned" about fighting near the Zaporizhzhia nuclear plant in southern Ukraine.
He made the comments during a summit with Ukraine's President Volodymyr Zelensky and Turkish leader Recep Tayyip Erdogan in Lviv.
"Any potential damage to Zaporizhzhia is suicide," Mr Guterres warned.
The meeting was the first between the UN chief and Mr Zelensky since Russia launched its invasion in February.
Mr Erdogan echoed the UN chief's concerns, telling reporters that he was worried about the danger of "another Chernobyl" disaster erupting at the plant
In recent weeks the area around the facility, which Russia seized in March, has come under heavy artillery fire, with both Kyiv and Moscow blaming each other for the attacks.
Ahead of Thursday's meeting Mr Zelensky criticised "deliberate" Russian attacks on the power plant.
Moscow is accused of turning the facility into an army base, with all three leaders urging the Russians to demilitarise the zone as soon as possible.
The appeals come as Ukrainian staff, who are working at the plant under Russian direction, warned of a potential nuclear catastrophe at the facility, saying in the past two weeks it has become "the target of continuous military attacks".
"What is happening is horrific and beyond common sense and morality," staff wrote in a Telegram post (in Ukrainian).
Later on Thursday, an official Twitter channel used by the Ukrainian government said that members of Rosatom, Russia's state nuclear corporation, had "urgently" left the facility, and an "unexpected day off" had been announced.
"Ukrainian intelligence officers believe that the Russians are preparing a provocation at the [facility]," Ukraine's Centre for Information security tweeted.
"Following their extensive shelling... [Russian forces] could 'raise the stakes' and stage a real terrorist attack on Europe's largest nuclear facility," it said.
Shortly before these tweets, Mr Zelensky warned that "the world is on a verge of nuclear disaster" and condemned what he called "Russia's irresponsible actions and nuclear blackmailing".