"We cannot allow the Iranian regime to use negotiations to stall for time, hedge its bets and keep open an indigenous route to a nuclear weapon," US national security adviser Stephen Hadley said in a speech.

"If there is one thing I hope we can all agree on, it is that a nuclear-armed Iran would be disastrous for the peace of the Middle East and the world," Hadley told representatives of some 80 countries.

He was speaking to an international gathering in Washington to mark five years since the founding of the Proliferation Security Initiative (PSI) that aims to curb the global spread of deadly weapons know-how.

Hadley also warned that North Korea still posed a proliferation threat and offered new details of a February 2007 incident in which four PSI partner countries thwarted a shipment of equipment to Syria that "could have been used to test ballistic missile components."

"A firm in one nation had manufactured the equipment. A firm in another nation was the intermediary that sold it to Syria. The shipping company was flagged in a third nation.

"And customs officials at the port of a fourth nation were alerted to offload and inspect the equipment and send it back to the country of origin," said Hadley, who did not name the countries involved.

"Interdictions like this one have been successful all over the world -- and have stopped many shipments of sensitive materials destined for Iran, North Korea, and Syria," he added.

"Yet North Korea and Iran remain great proliferation risks," said Hadley, who pushed Pyongyang anew to come clean on the range of its nuclear activities and dismantle its atomic infrastructure in line with a six-country agreement.

Hadley also warned that the United States would hold "fully accountable" any country, company, extremist group, or individual who helps US foes acquire weapons of mass destruction.

"The United States has made clear for many years that it reserves the right to respond with overwhelming force to the use of weapons of mass destruction against the United States, our people, our forces, and our friends and allies," he said.

"Today we also make clear that the United States will hold any state, terrorist group, or other non-state actor or individual fully accountable for supporting or enabling terrorist efforts to obtain or use weapons of mass destruction -- whether by facilitating, financing, or providing expertise or safe haven for such efforts," he said.

The United States and its European allies fear Iran wants to use the sensitive process of uranium enrichment to make an atomic weapon. Tehran insists its drive is entirely peaceful and has refused to freeze its efforts.

"So with many international partners, we will continue to turn up the pressure on the regime over its uranium enrichment activity -- with diplomatic isolation, implementation of UN sanctions, and with additional financial pressure," said Hadley.

The Islamic republic has thus far resisted three rounds of UN sanctions and rejected Washington's offer of negotiations if Tehran halts uranium enrichment.

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