President Barack Obama said Friday that the world faces a persistent and evolving threat from terrorists who are eager to unleash a devastating nuclear attack."It would change our world," he said.

Hosting his last nuclear security summit, said the world has measurably reduced the risk of nuclear terrorism by taking "concrete, tangible steps." He said no terrorists have thus far obtained nuclear material, and he praised recent moves by Argentina, Switzerland and Uzbekistan for getting rid of their stockpiles of highly enriched uranium.

Still, Obama said, the prospect of the Islamic State group or other extremists getting a weapon remains "one of the greatest threats to global security." He pointed out that IS had already used chemical weapons and that al-Qaida has long sought nuclear material.

"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," Obama said.

For the dozens of world leaders assembled in Washington this week, the harrowing risk of nuclear terrorism has been front and center, alongside concerns about North Korea's nuclear weapons program. Yet Obama worked to open the summit's final day on an optimistic note, hailing the nuclear agreement with Iran as a "substantial success" and a model for future diplomacy.

Obama sought to use the controversial Iran deal as an argument for his carrot-and-stick approach to deterring nuclear proliferation as he huddled with other U.N. Security Council members who negotiated the deal with the U.S. He credited Iran with taking steps to meet its commitments, though critics of the deal are livid about the sanctions relief Iran is receiving in response.

"It will take time for Iran to reintegrate in the global economy, but Iran is already beginning to see the benefits of this deal," Obama said.

Obama acknowledged that the nuclear deal hasn't swept away other issues the U.S. and other nations still have with Iran; support for terrorism and Tehran's ballistic missile program typically top that list. Still, he said all the nations that negotiated the deal could agree that it's been an effective way to address the narrower issue of nuclear proliferation in Iran.

"This is a success of diplomacy that hopefully we will be able to copy in the future," Obama said.

Frustration over the slow pace of reducing nuclear stockpiles has shadowed this year's summit, Obama's last major push on denuclearization. The absence of key players — especially Russia — further underscored the lack of unanimity still confronting global efforts to deter nuclear attacks.

After six years of prodding by Obama and others before him, the global stockpile of fissile material that could be used in nuclear bombs 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, readily available in hospital, industrial and academic settings. Concerns about substances like cesium or cobalt getting in the wrong hands have escalated sharply following deadly attacks by IS, raising the disturbing prospect of a nuclear attack on a Western city.

As the summit opened Thursday, leaders trained their focus on North Korea, whose continued provocations have stoked concerns throughout the region. Obama discussed steps to deter further North Korean missile tests during a meeting with Chinese President Xi Jinping. In another session with the leaders of Japan and South Korea, he called for vigorous implementation of stepped-up U.N. sanctions.

President Vladimir Putin of Russia, whose massive nuclear weapons stockpile is rivaled only by the U.S., refused to attend this year's summit. Moscow scoffed at what it deemed U.S. efforts to control the process and take power away from international agencies. Prime Minister Nawaz Sharif of Pakistan, another nuclear-armed country, canceled his trip following a bombing that killed 72.

-AP