SEOUL: North Korea’s military on Tuesday threatened to move back into zones that were demilitarized under inter-Korean peace agreements as the country continued to dial up pressure on rival South Korea amid stalled nuclear negotiations with the Trump administration.

The General Staff of the Korean People’s Army said it’s reviewing a ruling party recommendation to advance into unspecified border areas that had been demilitarized under agreements with the South, which would “turn the front line into a fortress.”

The powerful sister of North Korean leader Kim Jong Un days earlier said the North would demolish a “useless” inter-Korean liaison office in the border town of Kaesong and that she would leave it to the military to come up with the next step of retaliation against the “enemy” South.

“Our army is keeping a close watch on the current situation in which the (North-South) relations are turning worse and worse, and getting itself fully ready for providing a sure military guarantee to any external measures to be taken by the party and government,” said the KPA’s General Staff, which is akin to other countries’ Joint Chiefs of Staff.

It said it’s studying an “action plan for taking measures to make the army advance again into the zones that had been demilitarized under the (North-South) agreement, turn the front line into a fortress and further heighten the military vigilance against (the South),” according to the statement carried by the North’s official Korean Central News Agency.

While it wasn’t immediately clear what actions North Korea’s military might take against the South, the North has threatened to abandon a bilateral military agreement reached in 2018 to reduce tensions across the border.

The Koreas then committed to jointly take steps to reduce conventional military threats, such as establishing border buffers on ground and sea and no-fly zones. They also removed some front-line guard posts in a symbolic gesture. The North’s statement possibly implies that it would no longer respect the buffer zones and that the guard posts would be rebuilt.

The North’s military also said it would open unspecified areas near the ground border and its southwestern waters so that North Koreans could send anti-South Korea propaganda leaflets to the South, in an apparent tit-for-tat against North Korean defectors and activists floating anti-Pyongyang leaflets across the border.

This could potentially create security headaches for the South if North Korean military vessels escort North Korean civilian boats as they approach or cross the countries’ disputed western maritime border for leafleting, said Kim Dong-yub, an analyst from Seoul’s Institute for Far Eastern Studies and a former South Korean military official.

The waters have occasionally been the scene of bloody skirmishes, including a 2010 attack on a South Korean naval ship that killed 46 sailors. The North, which does not recognize the western maritime border drawn unilaterally by the U.N. at the close of the 1950-53 Korean War, has denied responsibility over the sinking of the Cheonan.

Choi Hyun-soo, spokeswoman of South Korea's Defense Ministry, said the South Korean and U.S. militaries were closely monitoring the North's military and that the inter-Korean military agreement should be kept.

Recently announced as her brother’s top official on inter-Korean affairs, Kim Yo Jong in recent weeks has repeatedly bashed South Korea over declining bilateral relations and its inability to stop leafleting by defectors and activists.

North Korea in recent months has suspended virtually all cooperation with the South while expressing frustration over the lack of progress in its nuclear negotiations with Washington, which have faltered with the Americans rejecting North Korean demands for major sanctions relief in exchange for a partial surrender of its nuclear capabilities. The North has also threatened to abandon bilateral peace agreements reached during Kim Jong Un’s three summits with South Korean President Moon Jae-in in 2018, while also expressing frustration over Seoul's unwillingness to defy U.S.-led international sanctions and restart inter-Korean economic cooperation.

Moon on Monday called on North Korea to stop raising animosities and return to talks, saying that the rivals must not reverse the peace deals.

Some experts say North Korea, which has mobilized people for massive demonstrations condemning defectors, is deliberately censuring the South to rally its public and shift attention away from a worsening economy. Although Kim Jong Un had declared a “frontal breakthrough” against sanctions, experts say the COVID-19 crisis likely thwarted many of his economic goals.