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 President Obama told a gathering of world leaders that the possibility of terrorists getting nuclear material and scientists would be catastrophic. 
 
 "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. 
 
 Related: Obama, World Leaders Discuss Keeping Nuclear Weapons from ISIS 
 
 Over the past two days, 50 world leaders have huddled on the best ways to keep terrorists from gathering materials for a dirty bomb and keeping nuclear weapons away from dangerous political regimes. The meeting comes on the heels of new information that some of the suspects in the Brussels' attacks were tracking and recording the movements of a top Belgian nuclear scientist. 
 
 Related: Brussels Attacks 
 
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 On Friday, Obama referenced those videotapes. 
 
 "It would change our world. We cannot be complacent. We have to build on our progress," the president said. 
 
 U.S. officials estimate there's some 2000 metric tons of material being housed in civilian and military programs around the world that could be used to craft nuclear weapons. In an op-ed published this week in the Washington Post President Obama underscored the need to work with world leaders in preventing terrorist groups like ISIS from getting that material. 
 
 "Given the continued threat posed by organizations such as the terrorist group we call ISIL, or ISIS, we'll also join allies and partners in reviewing our counterterrorism efforts, to prevent the world's most dangerous networks from obtaining the world's most dangerous weapons," Obama wrote. 
 
 The president met earlier on Friday with leaders from nations that worked, alongside the U.S., to reach a historic accord aimed at curbing Tehran's nuclear program. Those efforts, the president said, involved intense negotiations and are backed by strong sanctions. 
 
 Related: Fact Sheet: Who Has Nuclear Weapons, And How Many Do They Have? 
 
 "The countries represented in this room achieved what decades of animosity and rhetoric did not — a long-term deal that closes off every possible path to building a nuclear weapon, and subjects Iran to the most comprehensive nuclear inspections ever negotiated," Obama said. 
 
 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. 
 
 The administration cites the Iran nuclear deal and an arms reduction treaty with Russia as major steps toward exactly that type of non-proliferation. 
 
 But while the U.S. and Russia have an agreement to decrease their Cold War-era nuclear arsenals, Russia opted to skip this year's summit citing tension over Ukraine and Syria. Russia — along with the U.S. — holds a combined 90 percent of the world's nuclear weapons. 
 
 North Korea's claims to have tested a hydrogen bomb, followed by tests of a ballistic missile, are also provocations that the U.S. will continue to "deter and defend" again, Obama said on Thursday following a meeting with the leaders of Japan and South Korea, regional allies in that effort. 
 
 Some experts worry that such efforts aren't enough. 
 
 "The bad news is that for all their success, these summits are not moving fast enough. When fleeing a forest fire, direction is important — but so is speed," Joseph Cirincione, president of the Ploughshares Fund, a global security foundation wrote in a column for the Huffington Post. "Can we get to safety before catastrophe engulfs us?" ||||| Media playback is unsupported on your device Media caption President Obama: ''There's still a great deal of nuclear and radioactive material around the world that needs to be secured'' 
 
 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 were represented at the summit. 
 
 World leaders convening in Washington for the summit expressed concern about North Korea's nuclear weapons programme and Russia's lack of attendance. 
 
 Mr Obama expressed concern that Russia has been building up its military at the expense of nuclear arms reductions. 
 
 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. 
 
 Image copyright AFP Image caption Russia declined to the attend the summit in Washington 
 
 As the summit closed, the leaders reaffirmed their commitment to fight proliferation. 
 
 Mr Obama cited progress in making large parts of the world free of nuclear materials. South America had already achieved this, and Central Europe and South East Asia were expected to do so later this year. 
 
 "Together, we have removed the world's most deadly materials from nuclear facilities around the world," he said. 
 
 Despite these gains, Mr Obama said the Indian subcontinent and the Korean peninsula were areas where more could be done to combat proliferation. 
 
 Mr Obama said the world cannot be "complacent" and must build on its progress in slowing the stockpiling of nuclear weapons. 
 
 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." 
 
 Could Islamic State make a 'dirty bomb'? 
 
 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. 
 
 Mr Obama said Iran has so far followed the letter of the agreement. He urged patience as the country becomes re-integrated into the global economy. 
 
 "So long as Iran is carrying out its end of the bargain, we think it's important for the world community to carry out our end," he said. 
 
 At the summit, Mr Obama was asked about recent comments by presidential hopeful Donald Trump. 
 
 The Republican front-runner had floated the idea that Japan and South Korea should have nuclear weapons, a dramatic departure from decades of US foreign policy. 
 
 "The person who made the statement doesn't know much about foreign policy, nuclear policy, the Korean peninsula or the world generally," Mr Obama said. ||||| Canada's Prime Minister Justin Trudeau (L) speaks with U.S. President Barack Obama after the start of the second and final plenary session of the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jonathan Ernst 
 
 Dutch Prime Minister Mark Rutte (L) talks with China's President Xi Jinping during a family photo at the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jim Bourg 
 
 China's President Xi Jinping (L) speaks with Turkey's President Tayyip Erdogan (R) during the during first plenary session of the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jonathan Ernst 
 
 U.S. President Barack Obama (L) delivers remarks to start the final plenary session as he sits with (L-R) Britain's Prime Minister David Cameron, Lithuanian President Dalia Grybauskaite and Canada's Prime Minister Justin Trudeau at the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jonathan Ernst 
 
 U.S. President Barack Obama puts his hand on the shoulder of Netherlands Prime Minister Mark Rutte during the family photo of leaders at the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Kevin Lamarque 
 
 U.S. President Barack Obama opens the first opening plenary session as world leaders gather at the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jim Bourg 
 
 U.S. President Barack Obama (L) listens to remarks by Dutch Prime Minister Mark Rutte (R) at the opening of the first plenary session of the Nuclear Security Summit in Washington, April 1, 2016. REUTERS/Jonathan Ernst 
 
 U.S. President Barack Obama (R) smiles as he speaks with Dutch Prime Minister Mark Rutte (L) at the opening of the first plenary session of the Nuclear Security Summit in Washington, April 1, 2016. REUTERS/Jonathan Ernst 
 
 French President Francois Hollande (C) looks up as and other leaders take part in the first plenary session of the Nuclear Security Summit in Washington, April 1, 2016. REUTERS/Kevin Lamarque 
 
 Japanese Prime Minister Shinzo Abe (R) speaks with U.S. President Barack Obama during the first plenary session of the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Kevin Lamarque 
 
 U.S. President Barack Obama (C) hosts a meeting with members of the P5+1 group, with U.S. Secretary of State John Kerry (L), Britain's Prime Minister David Cameron (2nd L), French President Francois Hollande (3rd L) and Chinese President Xi Jinping (3rd R) attending, during the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Kevin Lamarque 
 
 World leaders gather for the opening plenary session of the Nuclear Security Summit in Washington April 1, 2016. REUTERS/Jim Bourg 
 
 Leaders convene for the first plenary session of the Nuclear Security Summit in Washington, April 1, 2016. REUTERS/Kevin Lamarque 
 
 U.S. President Barack Obama hosts a meeting with members of the P5+1 during the Nuclear Security Summit in Washington April 1, 2016. Flanking Obama are French President Francois Hollande (L) and Chinese President Xi Jinping. REUTERS/Kevin Lamarque 
 
 Leaders convene for the first plenary session of the Nuclear Security Summit in Washington, April 1, 2016. REUTERS/Kevin Lamarque 
 
 President Obama speaks with Dutch Prime Minister Mark Rutte at the opening of the first plenary session. REUTERS/Jonathan Ernst 
 
 Japanese Prime Minister Shinzo Abe checks his watch while seated with President Obama during the first plenary session. 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. “We cannot be complacent,” he said. 
 
 Obama said no group had succeeded in obtaining bomb materials but that al Qaeda had long sought them, and he cited actions by Islamic State 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.” 
 
 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. 
 
 Obama has less than 10 months left in office to follow through on one of his signature foreign policy initiatives. While gains have been made, arms-control advocates say the diplomatic process – which Obama conceived and championed - has lost momentum and could slow further once he leaves the White House in January. 
 
 A boycott by Russian President Vladimir Putin, unwilling to join in a U.S.-dominated gathering at a time of increased tensions between Washington and Moscow over Ukraine and Syria, may have contributed to summit results marked by mostly technical measures instead of policy breakthroughs. 
 
 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. Militants were found to have videotaped the daily routine of a senior manager of a Belgian nuclear plant, Obama said. 
 
 Obama said the required 102 countries have now ratified an amendment to a nuclear security treaty that would tighten protections against nuclear theft and smuggling. “We have measurably reduced the risks,” he said. 
 
 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 United States and Japan also announced they had completed the long-promised task of removing all highly enriched uranium and separated plutonium fuels from a Japanese research reactor. Japan is an avowedly anti-nuclear-weapons state as the only country to have ever suffered a nuclear attack. 
 
 Despite significant strides by Obama in persuading dozens of countries to rid themselves of bomb-making materials or reduce and safeguard stockpiles, much of the world's plutonium and enriched uranium remain vulnerable to theft. 
 
 Obama convened a separate meeting of world powers to take stock of the landmark nuclear pact they negotiated with Iran last July. It is a critical component of his nuclear disarmament agenda and a major piece of his foreign policy legacy. 
 
 LANDMARK PRAGUE SPEECH 
 
 Obama inaugurated the first Nuclear Security Summit nearly six years ago, after a 2009 speech in Prague laying out the goal of a world free of nuclear weapons. There is no guarantee that Obama's successor will keep the issue a high priority. 
 
 Obama made no public mention of Putin as a summit no-show. But he did say that because of the Russian’s leader’s emphasis on building up his military, there was unlikely to be any further deals for reducing the two countries’ vast nuclear weapons stockpiles during what is left of the Obama presidency. 
 
 For now, U.S. experts are less concerned about militants obtaining nuclear weapons than about thefts of ingredients for a low-tech dirty bomb that would use conventional explosives to disperse radioactive material and sow panic. 
 
 U.S. officials said they had no doubt that Islamic State, which controls swaths of Syria and Iraq, was interested in obtaining such materials, but authorities had no explicit evidence that the group had tried to do so. 
 
 Obama held a special summit session to coordinate the overall fight against Islamic State. He touted gains against the group in Iraq and Syria, which he said were forcing it to lash out elsewhere, and called for stepped-up efforts to stem the flow of foreign fighters to and from the battlefield. 
 
 Also looming over the summit was continuing concern about North Korea. Obama joined South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe on Thursday in vowing to ramp up pressure on Pyongyang in response to its recent nuclear and missile tests. 
 
 But So Se Pyong, North Korea's ambassador to the United Nations in Geneva, told Reuters on Friday that Pyongyang will pursue its nuclear and ballistic missile program in defiance of the United States and its allies, saying there is now a state of "semi-war" on the divided peninsula. 
 
 (Additional reporting by David Brunnstrom, Doina Chiacu, Patricia Zengerle, Lesley Wroughton, Idrees Ali, Timothy Gardner, David Alexander, Susan Heavey and Jeff Mason; Writing by Matt Spetalnick; Editing by James Dalgleish, Leslie Adler and Mary Milliken)
– 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.