US President Barack Obama warned of the dangers of nuclear terrorism in his opening remarks Friday at the Nuclear Security Summit in Washington.
"There's no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material, they most certainly would use it to kill as many innocent people as possible," Obama said.
Speaking to leaders representing some 50 countries who gathered for the fourth such summit, Obama said that protecting nuclear material represented "a 21st-century security challenge that no one nation can solve alone."
He called nuclear terrorism the greatest threat to the world's collective security.
Noting that "no terrorist group has succeeded thus far in obtaining a nuclear weapon" and that international efforts to date had "measurably reduced the risks" of nuclear material falling into the wrong hands, Obama pointed out the threat nevertheless "persists."
Al-Qaeda had sought nuclear material, and the attackers behind last week's Brussels bombings had videotaped nuclear plant employees, Obama said.
He pointed out that Islamic State forces have used mustard gas.
The closing hours of the two-day summit would concentrate on further collective measures to secure nuclear materials.
More than a dozen nations have removed all of their highly enriched plutonium and uranium, Obama said. Japan is currently removing a half ton of highly enriched nuclear material, the largest such project in history.
Obama said there was still 2,000 tonnes of nuclear material around the world, and "not all of this is properly secured." Just a small amount - "the size of an apple" - could kill and injure hundreds of thousands of people, he warned.
On the sidelines of the summit, China has agreed to work with the United States to intensify efforts to prevent smuggling of nuclear material across borders.
Speaking Thursday in Washington, Li Wei, China's deputy head for customs control, said that China will "through cooperation, improve its ability to prevent the illegal trafficking of nuclear and other radioactive substances."
Russia chose not to attend the summit.
Earlier Friday, Obama met with leaders of the P5+1 group, which negotiated the Iran nuclear deal. He thanked all the nations who participated in bringing the agreement to fruition.
"Thanks to this deal, we have seen real progress," Obama said.
He noted that it will "take time for Iran to reintegrate into the global economy, but Iran is already beginning to see the benefits of this deal."