That's after Seoul suspended part of a 2018 pact with the North a day earlier, in protest over Pyongyang's launch of a spy satellite earlier this week.
North Korea's defense ministry said in a statement:
"From now on, our army will never be bound by the September 19 North-South Military Agreement," and, "We will withdraw the military steps, taken to prevent military tension and conflict in all spheres including ground, sea and air, and deploy more powerful armed forces and new-type military hardware in the region along the Military Demarcation Line."
The statement came hours after Pyongyang fired a ballistic missile toward the sea on Wednesday.
South Korea said that launch appeared to have failed.
Tuesday's satellite launch was North Korea's third attempt this year, after two failures.
It came after North Korea's leader Kim Jong Un made a rare trip to Russia, during which Russian President Vladimir Putin vowed deeper cooperation, including helping Pyongyang build satellites.
Seoul says it's likely Russia helped in technical aspects of the launch, which drew condemnation worldwide for flouting UN resolutions that bar Pyongyang from tech that could be used in ballistic missile programs.
Seoul believes the satellite has entered orbit, but said it needs more time to work out if it was functional.
In response to the launch, South Korean Prime Minister Han Duck-Soo announced partially suspending the September 19 agreement, an inter-Korean military pact signed in 2018, until "mutual trust is restored" between the two Koreas.
One of the concrete steps is immediately resuming military observation of the North's forces in border areas.
The U.S. says Seoul's suspension of the deal was a "prudent and restrained response," citing Pyongyang's "failure to adhere to the agreement."
