The move comes in response to South Korea partially suspending the agreement and resuming surveillance flights along the border after North Korea claimed success in launching a spy satellite into space on Tuesday.
The launch sparked condemnation from the US, Japan, and South Korea, who said it flouted UN bans as it was done using ballistic missile technology.
Pyongyang vowed South Korea would “pay dearly” for its “irresponsible and grave” provocation in which it nullified some parts of the five-year-old pact, according to state media KCNA.
“From now on, our army will never be bound by the September 19 North-South Military Agreement,” said North Korea’s defence ministry in a statement.
“We will immediately restore all military measures that have been halted according to the north-south military agreement,” it said.
North Korea said it will now deploy stronger armed forces and “new-type military hardware” along the border.
It said South Korea will be held “accountable” in case of an irretrievable clash along the Military Demarcation Line, the heavily armed border between the two.
Pyongyang will withdraw all measures “taken to prevent military tension and conflict in all spheres including ground, sea and air”, it said.
North Korea fired a rocket in its third attempt to place its reconnaissance satellite into space and later said the launch was a success.
The North defended the launch and said it had a “legitimate” right to self-defence to “closely monitor and thoroughly cope with the enemies’ various military moves”.
South Korean officials said the latest launch most likely involved technical assistance from Russia.
North Korea and Russia had pledged to strengthen ties following Kim Jong-un’s visit to Russia in September. Both countries have denied Russian assistance in the launch.
In retaliation, South Korea suspended a part of the 2018 inter-Korean deal and resumed surveillance activities by deploying crewed and uncrewed reconnaissance aircraft in the border area on Wednesday, Yonhap news agency reported.
The defence pact known as the inter-Korean Comprehensive Military Agreement aimed at de-escalating tensions at the border.
It was signed in a 2018 summit between Mr Kim and then-South Korean president Moon Jae-in.
The deal aimed at preventing accidental clashes and created a 5km buffer zone, including a no-fly zone, to reduce the probability of an armed conflict.
South Korea’s defence ministry said the North violated the pact 17 times till last year, with 15 violations recorded just last year alone.
South Korean defence minister Shin Won-sik had earlier called for suspending the pact, saying it seriously limits their military’s aerial surveillance capabilities, reported Yonhap.
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