The US has unveiled a $450m initiative to stop potential terrorists building a "dirty bomb".
'Dirty bombs' can spread radioactive material over a wide area
US Energy Secretary Spencer Abraham said the Global Threat Reduction plan aimed to secure and dispose of nuclear material scattered across the world.
He said most of the money would go on safeguarding spent nuclear fuel from US and Russian programmes.
A dirty bomb is a device that uses conventional explosives to spread low-level radioactive material.
In a speech at the Vienna-based International Atomic Energy Agency (IAEA), Mr Abraham said "a global clean-out of those radiological materials that remain in the world" was a clear priority.
He said concerns had deepened since the recent uncovering of a Pakistani-led black market in nuclear technology and in the light of the 11 September attacks.
The initiative would focus on rounding up unused Russian highly enriched uranium (HEU) fuel scattered around the world , as well as spent nuclear fuel from the US and Russia.
Mr Abraham said it would aim to repatriate all unused Russian-origin HEU fuel by the end of next year and all spent nuclear fuel by 2010.
He said the initiative would also work to convert civilian research reactors HEU reactors to use low enriched uranium fuel instead - not just in the US but across the globe.
IAEA chief Mohamed ElBaradei said the US initiative was a "key step" in the fight against nuclear proliferation and "nuclear terrorism".
Weapons-grade HEU or plutonium extracted from spent fuel potentially can be used in nuclear weapons.