US President Barack Obama says the threat from terrorists trying to launch a nuclear attack that would 'change our world' is real.

He told a summit of world leaders in Washington that if the so-called Islamic State got a nuclear weapon it'd be 'one of the greatest threats to global security'.

102 countries have agreed to make their nuclear sites more secure

The US president told world leaders that Islamic State's video surveillance of a Belgian nuclear scientist was a clear statement of intent.