"Working together, our nations have made it harder for terrorists to get their hands on nuclear material," he told the Nuclear Security Summit in Washington DC.

Mr Obama said the expanded Convention on the Physical Protection of Nuclear Material would be effective soon.

More than 50 world leaders have gathered in the US capital over the past two days to discuss how to keep terrorists from gathering materials for a so-called dirty bomb, and other nuclear threats.

With Russian President Vladimir Putin boycotting the summit, a major deal on reducing global stockpiles of atomic weapons is out of reach.

On Friday, Mr Obama said the world has "measurably reduced" the risk from nuclear weapons.

But he warned there is a persistent and evolving threat of terrorists conducting a nuclear attack.

He mentioned evidence that extremists linked to the Paris and Brussels attacks had filmed a senior manager at a Belgian nuclear facility.

"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," Mr Obama told world leaders.

He warned that a bomb containing fissile material the size of an apple could be catastrophic.

"The smallest amount of plutonium could kill and injure hundreds of thousands of people," Mr Obama said. "It would be a humanitarian, political, economic, and environmental catastrophe with global ramifications for decades. It would change our world."