North Korean leader Kim Jong-un's sister Kim Yo Jong has erupted at South Korea, threatening military action.
Her anger exploded as she accused activists of sending anti-Pyongyang pamphlets across the border.
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She said South Korea was "the enemy" and warned of the collapse of the "useless" inter-Korean liaison office at the border.
She said North Korea's military would determine how to retaliate for the activists' propaganda leaflet campaign targeting North Koreans.
"By exercising my power authorised by the supreme leader, our party and the state, I gave an instruction to the arms of the department in charge of the affairs with enemy to decisively carry out the next action," she said in a statement carried by the North's official Korean Central News Agency.
"If I drop a hint of our next plan the [South Korean] authorities are anxious about, the right to taking the next action against the enemy will be entrusted to the General Staff of our army," she said.
"Our army, too, will determine something for cooling down our people's resentment and surely carry out it, I believe."
South Korea on Sunday convened an emergency security meeting and urged North Korea to uphold reconciliation agreements.
The meeting came hours after the North threatened to demolish a liaison office and take military action against its rival.
There's concern that North Korea could turn to provocation to bolster its internal unity and wrest outside concessions as nuclear talks with the United States remain deadlocked. Observers say North Korea desperately needs sanctions relief in the face of harsh US-led sanctions and the coronavirus pandemic.
South Korea's national security director, Chung Eui-yong, held an emergency video conference with ministers in charge of security and military generals on Sunday morning.
They discussed the latest situation on the Korean Peninsula and the government's possible steps, the presidential Blue House said in a statement.
The Unification Ministry, which handles relations with North Korea, later said that both Koreas must strive to abide by all agreements they have reached.
The Defence Ministry said separately it closely monitors North Korea's military and maintains a firm military readiness.
Both ministries said the South Korean government "views the current situation as grave".
North Korea earlier suspended communication lines with South Korea and threatened to nullify 2018 agreements that led the Koreas to halt firing exercises, remove some landmines and tear down guard posts in frontline areas.
The North has linked its recent series of threats to Seoul's failure to prevent activists from launching propaganda leaflets across their border.
But some experts say North Korea is deeply frustrated that South Korea hasn't done enough to revive lucrative joint economic projects as well as over a lack of progress in its nuclear talks with Washington.
It's still unclear if the North would go ahead with its threat to destroy the liaison office, which was built at a North Korean border town following a 2018 summit between Kim Jong-un and South Korean President Moon Jae-in.
Such a move could deepen anti-Pyongyang sentiments and make it difficult for the North to restore ties with South Korea when needed.