Spy satellites have spotted signs that North Korea may be preparing to transport another long-range missile to a test launch site, South Korean officials said Saturday, as the U.S. defense secretary issued his harshest warning to the North since its recent nuclear test.

"We will not stand idly by as North Korea builds the capability to wreak destruction on any target in Asia - or on us," Secretary of Defense Robert Gates told a regional defense meeting in Singapore. He said the North's nuclear program was a "harbinger of a dark future," but wasn't yet a direct threat.

Since last Monday's nuclear blast, North Korea has test-launched six short-range missiles in a show of force and announced it won't honor a 1953 truce ending fighting in the Korean War.

The reclusive communist state appears to be preparing to move a long-range missile by train from a weapons factory near Pyongyang to its northeastern Musudan-ni launch pad, a South Korean Defense Ministry official said. Images of the movements were captured by U.S. satellites, said the official, who was not allowed to be identified when discussing intelligence matters.

North Korea will need about two weeks to complete the launch preparation, South Korea's Yonhap news agency reported, citing an unidentified intelligence official. The missile might be tested around June 16 when the U.S. and South Korean presidents are to meet in Washington, Yonhap said.

Officials in Washington said Friday they noticed indications of increased activity at the missile test site, but did not provide many details. They spoke on condition of anonymity because methods of gathering information about North Korea are sensitive.

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