More than 50 North Korean submarines are apparently away from their bases for operations, a sign that the North is gearing up for combat while participating in high-level talks aimed at easing tension, an official here said Sunday.

"Seventy percent of North Korea's submarines left their bases, and their locations are not confirmed," the South Korean military official told reporters.

The North is known to have around 70 submarines.

The unpredictable communist nation has also doubled the number of its artillery troops on the border, with the command to be combat ready, according to the official.

"It's a very serious situation," the official said, saying it represents Pyongyang's typical "dual attitude."

The South's military, in close cooperation with the U.S., is doing its best to beef up the defense posture, he added.

Six South Korean fighter jets, deployed to take part in the Red Flag Alaska exercise, returned home Sunday ahead of schedule. The F-16s were initially scheduled to fly back to the country later this week.

As planned, meanwhile, top government officials from the two Koreas resumed their talks at the truce village of Panmunjom. (Yonhap)