Russian President Dmitry Medvedev has called for new international rules covering safety at nuclear plants.
Mr Medvedev was speaking as he and his Ukrainian counterpart, Viktor Yanukovych, made an unprecedented joint visit to the town of Chernobyl to mark the 25th anniversary of the world's worst nuclear accident.
A plume of radiation spread over parts of Europe from Chernobyl after a reactor there exploded during a safety systems test.
An international conference in Kiev last week promised millions of dollars for a new structure to seal off the shattered No 4 reactor.
At least 30 people were killed in the immediate aftermath of the explosion and a disputed number of others died later from radiation-related illnesses.