George Bush, the US president, has warned North Korea against peddling its nuclear weapons or materials to other countries, saying such a move would be "a grave threat to the United States and we would hold North Korea fully accountable for the consequences of such action".
North Korea conducted its first nuclear test on October 9, drawing widespread condemnation across the world.
Bush is in Hanoi for the Asia-Pacific Economic Co-operation summit.
He will meet world leaders over the weekend, including the leaders of Russia, Japan, China and South Korea – all partners with the US in talks aimed at persuading Pyongyang to abandon its nuclear weapons.
Said Bush: "For the sake of peace, it is vital that the nations of this region send a message to North Korea that the proliferation of nuclear technology to hostile regimes or terrorist networks will not be tolerated.
"In this new century America will remain engaged in Asia, because our interests depend on the expansion of freedom and opportunity in this region," adding that the US saw its role in Asia as one of "partnership, not paternalism".