REUTERS/Jim Bourg 
 
 WASHINGTON U.S. President Barack Obama urged world leaders on Friday to do more to safeguard vulnerable nuclear facilities to prevent “madmen” from groups like Islamic State from getting their hands on an atomic 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. Mr Obama said the world cannot be "complacent" and must build on its progress in slowing the stockpiling of nuclear weapons. "ISIL has already used chemical weapons — there is no doubt if these madmen ever got their hands on a bomb or nuclear material they would use it to kill as many as possible," Obama told world leaders meeting in Washington Friday for the Nuclear Security Summit. "It would change our world.” 
 
 Obama hosted more than 50 world leaders for his fourth and final summit focused on efforts to lock down atomic materials to guard against nuclear terrorism, which he called "one of the greatest threats to global security" in the 21st century. At the closing news conference, Obama, a Democrat, made clear that the raucous Republican presidential race, particularly controversial comments by party front-runner Donald Trump, weighed on leaders' discussions on the summit sidelines. Obama sternly dismissed as proof of foreign-policy ignorance Trump’s recent suggestion that Japan and South Korea should be allowed to build their own nuclear arsenals, putting him at odds with decades of U.S. policy. “The person who made the statements doesn't know much about foreign policy or nuclear policy or the Korean peninsula, or the world generally," Obama said, adding that Americans don’t want anyone with such views to occupy the White House. 'DIRTY BOMB' THREAT 
 
 Deadly bomb attacks in Brussels last month have fueled concern that Islamic State could eventually target nuclear plants, steal material and develop radioactive dirty bombs. But he acknowledged that with roughly 2,000 tons of nuclear material stored around the world, “not all of this is properly secured.” 
 
 Obama, wrapping up the summit, said leaders had agreed to strengthen their nuclear facilities against cyber attacks, something that outside experts see as a major weak point. The summit, and by extension Obama's efforts at shoring up his legacy on nuclear policy, began early in his presidency during a 2009 speech on the need for a global effort to secure nuclear material and atomic weapons. World leaders convening in Washington for the summit expressed concern about North Korea's nuclear weapons programme and Russia's lack of attendance.
– At a nuclear summit attended by more than 50 world leaders Friday in Washington DC, President Obama called the possibility of ISIS getting its hands on a nuclear weapon "one of the greatest threats to global security," the BBC reports. “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,” Obama said, per Reuters. “It would change our world.” It was part of Obama's pitch to world leaders to better secure their nuclear materials, which he said is the most effective way of keeping them from falling into the hands of terrorists. He also called on countries to stop stockpiling nuclear weapons. Obama said both al-Qaeda and ISIS are still seeking nuclear material for weaponizing, and the risk of nuclear terrorism remains very real. He pointed out recent news that ISIS members had been following a Belgian nuclear plant's senior manager and secretly recording his daily activities. "We cannot be complacent," he said. In addition to ISIS and al-Qaeda, world leaders also expressed concern with North Korea's nuclear weapons program. US officials estimate there is approximately 2,000 metric tons of material worldwide that could be used to build a nuclear weapon, NBC News reports. The US and Russia (Putin refused to attend the summit) account for 90% of the world's nuclear weapons with more than 14,000 nuclear warheads between them.