The North's military issued the menacing statement on Tuesday evening, angrily protesting annual military exercises South Korea is conducting jointly with the United States and international efforts to slap new sanctions on Pyongyang for its third nuclear test.
Kim Yong-chol, a hard-line North Korean general suspected of involvement in a series of provocations against the South, read the statement on state TV, saying the North "will completely declare invalid" the Armistice Agreement that ended the 1950-53 Korean War.
The North also said it will cut off a military phone line at the truce village of Panmunjom.
"There will be a defense ministry statement coming out on this," a presidential official said, adding that the North's statement can be taken as meaning that the North may attempt provocations because a "high-level person" read the statement.
"We're keeping a close watch over the situation," he said, adding that Pyongyang could issue a clearer position after the U.N. Security Council adopts a new sanctions resolution around Thursday.
The presidential office is handling the matter in the context of the National Security Council, and Kim Jang-soo, who has been named to head the national security office, is exercising oversight over the handling of North Korea issues, the official said.
President Park Geun-hye plans to establish the security office as part of a government reorganization to make it play the role of a "control tower" on security issues. But its formal establishment has been delayed as the reorganization bill is pending in parliament.
On Wednesday, senior presidential foreign affairs and security secretary Ju Chul-ki reported to presidential chief of staff Huh Tae-yeol that the government is taking firm control of the situation regarding North Korea, presidential spokesman Yoon Chang-jung said.
Kim, the nominee for the national security office chief, is also making the best effort to handle the situation, Yoon said, though Kim could not attend a meeting of senior presidential secretaries due to the parliamentary deadlock over the government reorganization bill.
North Korea issues harsh rhetoric when South Korea conducts joint military exercises with the U.S., denouncing them as a rehearsal for an invasion of the communist nation. Seoul and Washington have repeatedly said the drills are purely defensive.
The South's ruling Saenuri Party demanded the North stop its provocations against the international community.
"As the Kim Jong-un regime has provided the source of all problems, it must also take responsibility for them," Saenuri spokesman Lee Sang-il said in a statement. "North Korea shouldn't be the one taking issue with the U.N. Security Council and the annual South Korea-U.S. military drills when it hasn't even expressed remorse or apologized for its nuclear test."
Lee also warned that if North Korea continues such behavior, its Kim Jong-un regime will "forever remain as a bad regime."
"If North Korea conducts a military provocation against us, we will respond strongly and firmly," he added.
The South's main opposition Democratic United Party (DUP) condemned the North's threats and urged the communist nation to abandon its nuclear ambitions.
"We cannot hide our horror at North Korea apparently wanting to wage a war of mutual destruction on the Korean Peninsula," DUP spokesman Jung Sung-ho said in a statement. "North Korea brought the U.N.'s sanctions resolution upon itself with its nuclear test, and the North is responsible for all the consequences that follow."
Jung added that even China, North Korea's main ally, has tentatively agreed to the upcoming resolution, an indication that it too recognizes the North as "the problem."