President Barack Obama chats with Dutch Prime Minister Mark Rutte, next to Chinese President Xi Jinping , as they arrive for a “family photo” with world leaders attending the Nuclear Security Summit in Washington, Friday, April 1, 2016.

World leaders declared progress Friday in securing nuclear materials worldwide but warned of a persistent and harrowing threat: terrorists getting their hands on a nuclear bomb. “It would change our world,” President Barack Obama said.

Obama, addressing a nuclear security summit, said there was no doubt that if ISIS “madmen” obtained nuclear material, they would use it to kill as many people as possible. He urged fellow leaders not to be complacent about the risk of a catastrophe he said would have global ramifications for decades.

So far, no terrorists have gotten a nuclear weapon or a dirty bomb, Obama said, crediting global efforts to secure nuclear material. But he said it wasn’t for lack of trying: Al-Qaeda has sought nuclear materials, ISIS has deployed chemical weapons and extremists linked to the Brussels and Paris attacks were found to have spied on a top Belgian nuclear official, Obama said.

“We have measurably reduced the risk,” Obama said. Still, he added, “the threat of nuclear terrorism persists and continues to evolve.”

At this year’s summit – Obama’s last major push on denuclearization – deep concerns about nuclear terrorism have tempered other, more positive signs of the world coming together to confront the broader nuclear threat.

Obama spent part of the summit huddling with the leaders of South Korea and Japan about deterring nuclear-tinged provocations from North Korea, in a powerful show of diplomatic unity with two U.S. treaty allies. Similarly, Obama’s sit-down with Chinese President Xi Jinping offered the two strategic rivals a chance to illustrate their mutual concern about the North, a traditional Chinese ally.

Non-nuclear model

And the UN Security Council members who brokered a sweeping nuclear deal with Iran held up that agreement as a model for preventing nuclear proliferation, as they gathered on the summit’s sidelines to review implementation of the deal.

Aiming to show concrete action to strengthen nuclear security, leaders came to the nuclear summit with commitments in hand, known in diplomatic-speak as “gift baskets.” The White House spelled them out in a flurry of fact sheets.

Latin America and the Caribbean are now free of highly enriched uranium, the White House said, praising Argentina by name for converting its remaining stockpile into a less dangerous form. Fissile materials like highly enriched uranium and separated plutonium are necessary ingredients to make nuclear bombs.

The United States, in newly declassified statistics, said its own national inventory of highly enriched uranium has dropped from 741 metric tons two decades ago to 586 metric tons as of 2013. And the U.S. and Japan announced they’d finished removing hundreds of kilograms of weapons-grade material from a Japanese research reactor, and pledged to do the same at another.

On the global front, a strengthened nuclear security agreement was finally poised to take force, extending protections for nuclear materials being used, stored and transported while enacting new criminal penalties for nuclear smuggling. Those tweaks were approved in 2005, but have sat dormant awaiting ratification from a critical mass of nations, reached only in the past few days.

Still, frustration over the slow pace of reducing nuclear stockpiles shadowed the summit, Obama’s last major push on denuclearization. The absence of key players – especially Russia – further underscored the lack of unanimity confronting global efforts to deter nuclear attacks.

After six years of prodding by Obama and others before him, the global stockpile of fissile material remains in the thousands of metric tons. What’s more, security officials warn that the radioactive ingredients for a “dirty bomb” are alarmingly insecure in many parts of the globe.

Ahead of the summit, fewer than half of the countries participating had agreed to secure their sources of radiological material like cesium and cobalt, which are widely present in hospital, industrial and academic settings but could be diverted to make a dirty bomb. In the wake of Brussels and Paris, security officials have raised concerns that the next attack could spread cancer-causing substances across a wide swath of a Western city, wreaking havoc and triggering evacuations.

Obama has held four such summits in hopes of advancing the disarmament goals he set at the start of his presidency, when he declared in Prague that nuclear weapons were “the most dangerous legacy of the Cold War.” There are no clear indications they will continue after his presidency ends early next year.

“This summit is not the end of our quest to make the world safe from nuclear terrorism,”

Prime Minister Mark Rutte of the Netherlands said. He said the assembled leaders were passing the baton to international organizations. “Should the need arise, I know that everybody here will be ready to return.”

President Vladimir Putin of Russia, whose nuclear weapons stockpile is rivaled only by the stockpile in the U.S., declined to attend this year’s summit. Prime Minister Nawaz Sharif of Pakistan, another nuclear-armed country, canceled his trip following a bombing on Easter that killed 72 people.

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