The threat from terrorists trying to launch a nuclear attack that would “change our world” is real, President Barack Obama has said.
The world has taken “concrete” steps to prevent nuclear terrorism, he told the Nuclear Security Summit in Washington.
But the so-called Islamic State (IS) obtaining a nuclear weapon is “one of the greatest threats to global security,” he added.
More than 50 nations are represented at the summit.
IS has already used chemical weapons in Syria.
“There is no doubt that if these mad men ever got their hands on a nuclear bomb or nuclear material, they would certainly use it to kill as many people as possible,” he said.
“The single most effective defence against nuclear terrorism is fully securing this material so it doesn’t fall into the wrong hands in the first place.”
He said the world cannot be “complacent” and must build on its progress in slowing the stockpiling of nuclear weapons.
Mr Obama hailed his own nuclear agreement with Iran, which has been criticised by Republicans and other groups for the sanctions relief it provides. He called it a “substantial success”.
“This is a success of diplomacy that hopefully we will be able to copy in the future,” he said.
World leaders convening in Washington for the summit expressed concern about North Korea’s nuclear weapons programme, and Russia’s lack of attendance exemplified the struggle of worldwide unity on preventing nuclear attacks.
Russian President Vladimir Putin refused to attend the summit, and Prime Minister Nawaz Sharif of Pakistan cancelled his trip after the deadly bombing in Lahore. Both countries are nuclear-armed.