US President George W Bush has called for co-ordinated international action to curb the spread of nuclear weapons.
North Korea has admitted developing nuclear weapons
In a speech at the National Defense University in Washington, he demanded a strengthening of international treaties and the UN nuclear watchdog.
This would include a limit on the number of nations allowed to produce nuclear fuel.
Mr Bush said nuclear weapons in the hands of terrorists and "outlaw regimes" were a threat to world peace.
His comments came in the wake of a major scandal involving a leading Pakistani scientist.
The "father" of Pakistan's nuclear bomb, Dr Abdul Qadeer Khan, has admitted supplying nuclear technology to North Korea, Iran and Libya through a black market.
The BBC's Rob Watson, in Washington, says the timing of the speech was decidedly political, coming from a president seeking to regain the political initiative in the vital area of national security.
Our correspondent says this was radical stuff from Mr Bush, proposing nothing less than the unpicking of the 30-year-old bargain between the nuclear haves and have-nots.
The president said international treaties intended to regulate the development of nuclear power needed to be strengthened to stop countries producing material which could be used for weapons.
North Korea and Iran had both done this by exploiting loopholes allowing the enrichment and reprocessing of uranium for peaceful purposes, he said - and this had to be stopped.
He called on the 40 countries of the Nuclear Suppliers Group, which sell nuclear technology, to refuse to sell equipment to any country not already equipped to make nuclear fuel.
The president also criticised the way the International Atomic Energy Authority operated, saying countries like Iran, which were suspected of breaking the rules, should not be allowed to sit on the committees that enforced them.
And he urged law enforcement agencies - including Interpol - to join the battle to prevent the illegal movement of nuclear technology and materials.
Khan publicly admitted his role in trading nuclear information
Mr Bush said Iran and North Korea's nuclear programmes were a threat to the US and her allies, and insisted that the toppling of Saddam Hussein's regime in Iraq had prevented the proliferation of weapons of mass destruction.
He praised the decision of the Libyan leader, Colonel Gaddafi, to abandon the pursuit of WMDs and said he expected other countries to follow his lead or face international isolation, economic hardship and "other unwelcome consequences".
"The greatest threat before humanity today is the
possibility of secret and sudden attack with chemical or
biological or radiological or nuclear weapons," said Mr Bush.
"We must confront the danger with open eyes and unbending
"I've made clear to all the policy of this
nation: America will not permit the terrorists and
dangerous regimes to threaten us with the world's most