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.

US President Barack Obama urged world leaders on Friday to do more to safeguard vulnerable nuclear facilities to prevent “madmen” from groups like ISIS 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 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.”

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.

‘Dirty bomb’ threat

Deadly bomb attacks in Brussels last month have fueled concern that ISIS 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.”

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