UPDATED 7.25PM Barack Obama says the threat of a nuclear attack is real, but concrete steps are being taken to prevent nuclear terrorism.

The US President addressed Prime Minister John Key and more than 50 other world leaders at an international summit on nuclear security today in Washington DC.

Mr Obama said there is a real possibility terrorist groups like Islamic State could get access to nuclear weapons.

"There is no doubt that if these mad men ever got their hands on a nuclear bomb or a nuclear material, they most certainly would use it to kill as many innocent people as possible," he said.

"There's still roughly 2000 tonnes of nuclear materials, and not all of this is properly secured. And just the smallest amount of plutonium, about the size of an apple, could kill and injure hundreds of thousands of innocent people."

But he said more than 100 countries have now signed up to a pact aimed at stopping terrorists getting their hands on nuclear material, and important steps are being taken to prevent this happening.

"And that includes improved physical security, stronger regulations, abiding by international guidelines, greater transparency, strengthening our nuclear facilities, defences against cyber attacks."

Prime Minister John Key is there, despite New Zealand's well-publicised nuclear-free status. 

He said it's an important issue for the country, even though we don't have nuclear power and we don't have nuclear weapons.

Mr Key told Jack Tame our nuclear free status carries some weight.

"The fact that we went anti-nuclear and then got invited to the first nuclear summit on the basis that we were anti-nuclear had a degree of irony about it. But it also represents the way that New Zealanders are thought of."

However, Mr Key explained that "for some unique, specific scientific experiments very small amounts of uranium are used."

But, he said, it's very small quantities and it's not enriched uranium.

"If it fell into the wrong hands, it still presents a risk."