President George W Bush has called for an international effort to stop the global spread of weapons of mass destruction, describing them as the "the greatest threat before humanity."
In a speech in Washington, he said September 11th had "raised the prospect of even worse dangers".
"The greatest threat before humanity today is the possibility of secret and sudden attack with chemical, or biological or radiological or nuclear weapons," he said.
Mr Bush was speaking at the National Defence University at Fort McNair.
He said terrorists were now willing to use chemical, biological or nuclear weapons as a first resort.
"We saw the great harm that a stateless network could inflict upon our country. Killers armed with boxcutters, mace and 19 airline tickets," he said.
He said weapons of mass destruction could spread from two sources - rogue states, and black market dealers motivated by "greed, or fanaticism or both."
He said: "The former dictator of Iraq possessed and used weapons of mass destruction against his own people.
"For 12 years he defied the will of the international community. He refused to disarm or account for his illegal weapons and programmes.
"He doubted our resolve to enforce our word, and now he sits in a prison cell while his country moves toward a democratic future."
Mr Bush was speaking after this week's disclosure that Pakistani scientist Abdul Qadeer Khan sold nuclear secrets to states including Iran, Libya and North Korea.
The scientist, known as the father of Pakistan's nuclear programme, was pardoned by the country's President Pervez Musharraf.
Mr Bush's speech will be seen as a further attempt to lean on Pakistan to shut down the black market which has operated from within.