A defense ministry spokesman said 70 percent of the North’s total submarine fleet — or around 50 vessels — had left their bases and disappeared from Seoul’s military radar.
The movement of such a large number of submarines was “unprecedented,” the spokesman said, adding that Seoul and Washington were beefing up their military surveillance in response.
The North has also doubled the number of artillery units along the heavily-fortified land border with the South, he said.
The move came as top officials from both Koreas resumed a talks aimed at easing military tensions after a marathon negotiating session the night before ended without final agreement.
“The North is adopting a two-faced stance with the talks going on,” said the spokesman.
Yonhap news agency, citing military officials, said the submarine deployment was the largest since the end of the 1950-53 Korean War.
“The number is nearly 10 times the normal level … and it is a very serious situation,” it quotes one unnamed official as saying.
Tension flared on the Korean peninsula after Seoul accused Pyongyang of planting landmines across the border that earlier this month maimed two South Korean soldiers.
Pyongyang denied involvement but Seoul retaliated by resuming loudspeaker propaganda broadcast hated by the North along the border on August 10.
The North’s leader Kim Jong-Un last week ordered his military to move to a war-footing after an exchange of artillery fire on Thursday that claimed no casualties but further escalated tension.
This undated picture released from North Korea's official Korean Central News Agency (KCNA) on June 16, 2014 shows North Korean leader Kim Jong-un (left in black) inspecting the submarine No. 748 of Korean People's Army (KPA) naval unit 167 led 7th regiment at an undisclosed location in North Korea.