North Korea has doubled its artillery power and the majority of its submarines seem to be away from their bases, a South Korean military official said, as cited by the Yonhap news agency. The announcement comes amid ongoing talks between the two sides over the border crisis.

According to the official, more than 50 North Korean submarines are out for operations.

"Seventy percent of North Korea's submarines left their bases, and their locations are not confirmed," a South Korean military official said, adding that Pyongyang has about 70 submarines in total.

50 N. Korean submarines away from base http://t.co/5DqHXZ5KTz — Yonhap News Agency (@YonhapNews) August 23, 2015

"It's a very serious situation," the official added.

In the meantime, the Yonhap news agency reported that six South Korean fighter jets have returned home ahead of schedule from the Red Flag Alaska military drills. The F-16s were initially planned to fly back later this week.

Tensions have recently escalated on the Korean peninsula after both sides exchanged fire on Thursday. Also on Thursday there were reports that South Korea had ordered the evacuation of some 15,000 civilians from the border area to the west of the Korean Peninsula, reportedly shelled by the North’s military.

S. Korea, US fly 8 fighter jets ‘to show off force’ amid escalating crisis with Pyongyang http://t.co/czwP2fDpa4pic.twitter.com/k6PVBCvQxw — RT (@RT_com) August 22, 2015

On Saturday, Pyongyang delivered an ultimatum to Seoul demanding it stop broadcasting propaganda via loudspeakers across the borders. It set a deadline of 17:00 local time (08:00 GMT) on Saturday for this to happen. The North threatened ‘imminent’ military action if Seoul didn’t meet the demands.

However, South Korean Vice Defense Minister Baek Seung-joo told parliament that the broadcasts would continue, adding the North was likely to fire at areas in the Demilitarized Zone where transmitters are stationed.

READ MORE: S. & N. Korea trade warmongering accusations following exchange of fire

The second round of talks between the North and South’s top officials are ongoing on Sunday. Earlier on Saturday, the sides started negotiations to try and ease the border crisis. The meeting lasted over 10 hours and went past midnight. The sides then adjourned to review each other's positions.

READ MORE: Koreas ‘on brink of war’: Pyongyang marshals troops after exchange of fire

According to a South Korean Defense Ministry official, Seoul will target any Pyongyang units that attack loudspeakers in the Demilitarized Zone.

"In case of fire on broadcasting stations in the Demilitarized Zone, our armed forces will counterattack" he told TASS, adding the South’s military “would act proportionally against any threats they face.”

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