PANMUNJOM, Korea — 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," Defense Secretary 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 the 1953 truce that ended the fighting in the Korean War.

Now, 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.