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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

On Friday, Obama referenced those videotapes.

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"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?"