North Korea has drastically shortened the time it takes to prepare for missile launches by improving liquid fuel quality for ballistic missiles, a government source here claimed Monday.

This could virtually incapacitate South Korea's current missile defense system.

"It used to be thought possible to detect a North Korean launch of ballistic missiles in advance because the liquid fuel had to be pumped into the missiles right before the launch," the source said.

"But analysis of various intelligence reports last year shows that the North's ballistic missiles can now stay in standby mode for a long time even after they are injected with liquid fuel because its quality has improved."

That would give the renegade country more flexibility in deciding when to launch a missile.

The North is estimated to have some 1,000 ballistic missiles, most of which use liquid fuel.

In the past the North's liquid fuel was so explosive that missiles could only be filled an hour-and-a-half to three hours before their launch. But now they can be kept ready for anywhere between a month and several years, the source claimed.

That would make i t nearly impossible for South Korean and U.S. intelligence to spot launch preparations with reconnaissance satellites or aircraft.

Military authorities here failed to detect the North's surprise launches of Scud and Rodong missiles in advance last year. They are now trying to improve their reconnaissance and responses.

