South Korea's presidential office has condemned the powerful sister of North Korean leader Kim Jong Un for making rude and "senseless" remarks against President Moon Jae-in, in a heated war of rhetoric following Pyongyang's unilateral destruction of an inter-Korean liaison office.

Cross-border tension escalated yesterday as the belligerent North threatened to redeploy troops to two inter-Korean zones, rebuild demolished guard posts, and resume military exercises along the border.

These acts would nullify an inter-Korean military agreement signed in September 2018 to reduce tension along the border.

The regime also refused to receive special envoys from the South.

"We warn that we will no longer tolerate North Korea's reckless words and actions," South Korea's presidential spokesman Yoon Do-han said at a briefing, adding that the moves harm mutual trust between their two leaders.

He was referring to a statement by Mr Kim's sister Yo Jong, which criticised Mr Moon's conciliatory speech on Monday to mark 20 years of inter-Korean relations.

She described his remarks as "shameless sophistry" filled with "flowery rhetoric" and accused him of "pro-US flunkeyism" - the latest in a series of insults she hurled towards the South since early this month for allowing North Korean defectors to release anti-Pyongyang propaganda balloons across the border.

Mr Yoon also chided Pyongyang for its "unprecedentedly senseless act" of saying that Mr Moon had "begged" the regime to receive his special envoy - either national security director Chung Eui-yong or National Intelligence Service director Suh Hoon.

Separately, the South Korean military warned that the North will "pay the price" if it carried out its military threats, adding that the moves would "thwart two decades of efforts by South and North Korea to improve inter-Korean ties and maintain peace on the Korean peninsula".

In a statement, the Joint Chiefs of Staff also said they are closely monitoring North Korea's military moves round the clock and maintaining combat readiness. "We will continue to manage the situation and prevent it from escalating into a military crisis," the statement said.

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This article was adapted from The Straits Times