U.N. Secretary-General Ban Ki-moon is encouraging the world to learn from past nuclear accidents such as the April 1986 nuclear disaster in Chernobyl.

At a conference in Kiev, Urkraine marking the 25th anniversary of the Chernobyl disaster, the U.N. head said that past nuclear accidents are posing the painful question of whether the risks and costs of nuclear development have been accurately calculated.

Ban said that the unfortunate truth is that the world will probably see more similar disasters.

He added that nuclear safety standards should be strengthened both at national and international levels. He also called for greater attention to be paid to the impact natural disasters can have on nuclear power plants.

The U.N. chief visited the nuclear accident site in Chernobyl and also had a 20-minute meeting with Ukrainian President Viktor Yanukovych.