US President Barack Obama told a global nuclear security summit Friday that the world had made progress in preventing terror groups like Al Qaeda and IS from acquiring nuclear weapons, but more needs to be done.

US President Barack Obama told a global nuclear security summit Friday that the world had made progress in preventing terror groups like Al Qaeda and IS from acquiring nuclear weapons, but more needs to be done.  

"By working together our nations have made it harder for terrorists to get their hands on nuclear material. We have measurably reduced the risk," he told leaders gathered in Washington.

But he also warned that thousands of tonnes of fissile material lies in stockpiles, under sometimes patchy security, and that material the size of an apple could cause devastation that would change the world.