SINGAPORE (Reuters) - U.S. Defense Secretary Robert Gates said on Saturday the United States would not accept a nuclear-armed North Korea and he warned Pyongyang against transferring nuclear material overseas.

A South Korean newspaper reported that Pyongyang was preparing to move an intercontinental ballistic missile from a factory near the capital to a launch site on the east coast.

In a speech to the Asia Security Conference in Singapore, Gates said the threat from North Korea, which this week detonated a nuclear device and launched a series of missiles, could start 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,” he said. “We will not accept North Korea as a nuclear state.”

North Korea -- one of the world’s last remaining Communist states -- has said it was no longer bound by the armistice that ended the 1950-53 Korean War. It threatened further actions in response to any U.N. censure.

Gates said Washington would hold North Korea accountable if it transferred any nuclear material outside its borders.

Impoverished North Korea has earned billions of dollars from exporting missile technology to Pakistan and the Middle East, defense analysts say.

Gates did not elaborate on how the United States might respond. He had earlier said no additional troops were being deployed to the peninsula, where 28,000 U.S. soldiers are stationed, and he emphasized diplomacy in his remarks.

HEIGHTENED ALERT

South Korean and U.S. troops were on heightened alert over the possibility that Pyongyang may provoke an incident along their heavily armed border.

North Korea has said it might test an intercontinental ballistic missile in response to U.N. 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,” South Korea’s Dong-a Ilbo newspaper quoted a source in Washington as saying.

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

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South Korea’s defense ministry had no immediate comment.

A U.S. defense official said the United States had observed “above average activity” at a site in North Korea that has previously been used to test-fire long-range missiles.

In New York, the United States and Japan circulated a draft U.N. Security Council resolution condemning the claimed nuclear test and demanding strict enforcement of sanctions imposed after the North’s first atomic test in October 2006.

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In Moscow, the Kremlin said Russian President Dmitry Medvedev and Japanese Prime Minister Taro Aso agreed on the need for a serious response to North Korea.

“The parties shared the view that there is a need to most seriously respond to these steps, representing a challenge to the international security system,” a Kremlin statement said.

Gates said North Korea was not a direct military threat now.

“If (the North Koreans) continue on a path they are on, I think the consequences for stability in the region are significant and I think it poses the potential for some kind of an arms race in this region,” Gates said.

He met with his South Korean and Japanese counterparts on the sidelines of the Singapore conference.

Western diplomats said permanent Security Council members Russia and China have agreed in principle that North Korea should be sanctioned for its nuclear test but it was not clear what kind of penalties they would support. Both are generally reluctant to approve sanctions.

“Our hope is that all parties concerned will remain cool-headed and take measures to address the problem,” Ma Xiaotian, The Chinese army’s deputy chief, told the conference.

“Our view is that the Korean peninsula should move toward denuclearization.”

U.S. officials have urged China to pressure North Korea to step back from nuclear brinkmanship and return to stalled disarmament talks. But many Chinese analysts say Washington overstates Beijing’s sway over Pyongyang, as well as their government’s willingness to use that influence.

North Korea was celebrating the nuclear test in rallies and meetings across the country, “highly proud and honored with the status of the country as a nuclear weapons state,” the official KCNA news agency said.