US President Barack Obama has urged world leaders to do more to safeguard vulnerable nuclear facilities to prevent "madmen" from groups like Islamic State (Isis) from getting their hands on a nuclear weapon or a radioactive "dirty bomb".
Speaking at a nuclear security summit in Washington, Obama said the world faced a persistent and evolving threat of nuclear terrorism despite progress in reducing such risks. "We cannot be complacent."
He said no group had succeeded in obtaining bomb materials but that al-Qaeda had long sought them, and he cited actions by Isis militants behind recent attacks in Paris and Brussels that raised similar concerns.
"There is no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material, they would certainly use it to kill as many innocent people as possible," he said. "It would change our world."
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Obama hosted more than 50 world leaders for his fourth and final summit focused on efforts to lock down vulnerable atomic materials to prevent nuclear terrorism, which he called "one of the greatest threats to global security" in the 21st century.
North Korea's nuclear defiance was also high on the agenda.
Obama has less than 10 months left to follow through on one of his signature foreign policy initiatives.
Although gains have been made, arms-control advocates say the diplomatic process - which Obama conceived and championed - could slow further once he leaves the White House in January.
A boycott by Russian President Vladimir Putin, unwilling to join in a US-dominated gathering at a time of increased tensions between Washington and Moscow over Ukraine and Syria, may have contributed to summit results that are mostly technical measures instead of policy breakthroughs.