South Korea's military has strengthened its monitoring of North Korea, but no unusual moves have been detected so far, a source said Monday, after Pyongyang threatened to take military action against the South.

In a statement Saturday, North Korean leader Kim Jong-un's powerful sister, Kim Yo-jong, said its military will be entrusted with the right to take the "next step" against the South in the latest in a series of threats made by the North in anger over anti-Pyongyang leaflets sent across the border by activists here.

Amid concerns over the North's possible provocation at the inter-Korean maritime border or near the Demilitarized Zone separating the two Koreas, the Joint Chiefs of Staff (JCS) said there have been no unusual activities by the North to report on.

"We are closely monitoring North Korean military's moves and are maintaining a firm military readiness posture. As of yet, there has been no incident to give additional explanations on," JCS spokesperson Col. Kim Jun-rak told a regular press briefing.

A source said the military is keeping close tabs on the North with its surveillance assets in the front line, as well as in the air and on the waters.

"The monitoring has been heightened overall," the source said.

Following the North's threat, the defense ministry said Sunday it is maintaining a staunch readiness posture to respond to all situations involving North Korea, calling on Pyongyang to comply with the inter-Korean military pact signed in 2018 to halt all hostile acts against each other. (Yonhap)