"As president, my number one security goal will be to prevent the terrorists from gaining weapons of mass murder," Kerry said in a speech at West Palm Beach, Florida.

The veteran Democratic senator criticized his Republican adversary, US President George W. Bush, for having "done too little, often too late," following the September 11, 2001, attacks to counter the risk of nuclear terrorism.

"It is time again that we have leadership at the highest levels that treat this threat with the sense of seriousness, urgency and purpose it demands," the Massachusetts senator said.

He underlined that the United States would need the support of the international community to face this challenge.

"We must use the might of our alliance," he said.

"We need to employ a layered strategy to keep the worst weapons from falling into the worst hands," Kerry said.

America, he stressed,"must lead and build an international consensus for early preventive action."

He said it is first necessary to gather up any rogue nuclear materials around the world that could be used to make bombs.

He added that he would seek to conclude a rapid accord with Russia for securing its nuclear stocks, but stressed that the threat of rogue material is not limited to the former Soviet Union.

Kerry also proposed a total ban on the production of enriched uranium and plutonium that could be used to build nuclear weapons.

The Democratic candidate also said he was against the development of bunker-busting thermonuclear bombs that could penetrate deep underground. Such devices have been backed by the Bush administration.

The Bush administration had been "fixated" on Iraq, while the nuclear dangers from North Korea and Iran had grown, he charged.

"We must also be prepared to talk directly with North Korea," he said, adding "we must be prepared to negotiate a comprehensive agreement that addresses the full range of issues of concern to us and our allies."

Kerry said a nuclear armed Iran would be "unacceptable."

If elected, Kerry said he would create a new post in the White House of a national coordinator for nuclear terrorism and counterproliferation.

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