Play Facebook 
 
 Twitter 
 
 Embed Obama: ISIS 'Madmen' Would Use Nuclear Weapons for Mass Murder 1:47 autoplay autoplay Copy this code to your website or blog 
 
 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 
 
 Play Facebook 
 
 Twitter 
 
 Embed Obama: We Must Avoid 'Madmen' Terrorists Getting Nukes 1:06 autoplay autoplay Copy this code to your website or blog 
 
 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. ||||| President Barack Obama called Wednesday for “peace with justice” as he proposed reducing U.S. nuclear arsenals by as much as one-third in a wide-ranging call for action delivered on the eastern side of Berlin’s Brandenburg Gate. 
 
 “Our work is not yet done. For we are not only citizens of America or Germany, we are citizens of the world,” Obama said, drawing on John F. Kennedy’s 1963 speech on the west side of the the city, in which he called on Berliners to look ahead to “the day of peace with justice” and to the rest of mankind. “We may not live in fear of nuclear annihilation — but as long as nuclear weapons exist, we are not truly safe.” 
 
 Text Size - 
 
 + 
 
 reset Obama in Germany, Belfast Play Slideshow 
 
 He said the United States is now looking to do more to reduce that threat. “After a comprehensive review, I have determined that we can ensure the security of America and our allies and maintain a strong and credibility strategic deterrent while reducing our deployed strategic nuclear weapons by up to one-third,” he said. “I intend to seek negotiated cuts with Russia to move beyond Cold War nuclear postures” and to work with NATO allies for “bold reductions in Europe.” 
 
 “We can forge a new international framework for peaceful nuclear power, reject the nuclear weaponization that North Korea and Iran may be seeking,” he added. 
 
 Obama has directed the Defense Department to strengthen its non-nuclear capabilities, to reduce the role of nuclear weapons in deterring non-nuclear attacks, and to reduce the role of nuclear launches in contingency planning. 
 
 New START calls for the United States and Russia to cut their arsenals to 1,550 nuclear warheads by 2018. Obama’s proposal, which would bring that number down to about 1,000, comes as tensions between the United States and Russia over Syria have been on stark display — most recently on Monday, as Obama and Russian President Vladimir Putin delivered a cold joint statement at the G8 summit, and appeared to make little progress on the issue. 
 
 In another big speech in a major European city, in Prague during the spring of 2009, Obama set an even more ambitious long-term goal: “a world without nuclear weapons.” It’s a hope he’s reiterated since, including at last year’s Nuclear Security Summit in Seoul, and one in which he’s said he still believes. 
 
 Obama spent much of his speech Wednesday leaning on Kennedy’s “Ich bin ein Berliner” speech given 50 years ago next week, as he laid out a vision in which the United States and Germany “pursue peace and justice not only for our countries, but for all mankind.” 
 
 While remaining “vigilant” about terror threats, “we must move beyond the mindset of perpetual war,” Obama said, echoing the philosophy he laid out last month in his speech on counterterrorism in the post-post-9/11 era. In the United States, “that means redoubling our efforts to close the prison at Guantanamo,” he said to cheers.
– 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.