US Defence Secretary Robert Gates said today that the United States would not accept a nuclear-armed North Korea and sternly warned it against transferring any nuclear material.

In a speech to an Asian defense conference in Singapore, Gates also said the threat from North Korea, which this week detonated a nuclear device and launched a series of missiles, could trigger an arms race in Asia.

"We will not stand idly by as North Korea builds the capability to wreak destruction on any target in the region or on us," Gates said. "We will not accept North Korea as a nuclear state."

Compounding tensions on the Korean peninsula, a South Korean newspaper quoted a source in Washington as saying Pyongyang was preparing to move an intercontinental ballistic missile from a factory near the capital to a launch site on the east coast.

Increasingly belligerent North Korea has warned of war, saying it was no longer bound by an armistice that ended the 1950-53 Korean War and threatened further provocations in response to UN Security Council censure.

Gates said the Obama administration would hold North Korea "fully accountable" if it transferred any nuclear material outside its borders.

"The transfer of nuclear weapons or material by North Korea to states or non-state entities would be considered a grave threat to the United States and our allies. And we would hold North Korea fully accountable for the consequences of such action."

The statement seemed to harden and broaden the Obama administration's stand on North Korea's recent series of provocations from a regional security issue to a global proliferation threat.

Regional powers are waiting to see what the North might do next after it conducted a nuclear test on Monday. South Korea is on alert on the assessment Pyongyang may make provocative moves using conventional weapons at their heavily armed border.

North Korea has warned of an intercontinental ballistic missile test in anger over U.N. Security Council punishment for what Pyongyang said was a satellite launch on April 5.

"Preparations to move an ICBM from the Saneum Weapons Research Center near Pyongyang by train have been captured by U.S. spy satellites," Saturday's Dong-a Ilbo newspaper quoted a source in Washington knowledgeable about the issue as saying.

The research lab is the North's main centre of research and manufacture of long-range missiles, the newspaper said.

South Korea's defence ministry could not immediately comment on the report.

• Signs Pyongyang prepping long-range missile