The US-led UN Command (UNC) said Thursday it is investigating a series of incidents at the inter-Korean border earlier this week, including a brief border incursion by North Korean troops and South Korea's resumption of anti-Pyongyang broadcasts.

On Sunday, some 20 North Korean soldiers crossed the Military Demarcation Line inside the Demilitarized Zone (DMZ), separating the two Koreas, in the central section of the border before retreating northward after the South fired warning shots, according to Seoul's military.

The incursion came just hours before the South blared anti-Pyongyang broadcasts toward the North for the first time in six years in response to the North's recent launches of trash-carrying balloons.

"We take our mission seriously at the United Nations Command and are currently investigating the recent issues with utmost diligence," the UNC said when asked about the incidents.

"Our actions are in strict accordance with the Armistice Agreement as we work towards deescalating the situation to ensure peace and stability in the region. We continue to call on the DPRK to return to dialogue using our established mechanisms."

DPRK stands for the North's official name, the Democratic People's Republic of Korea.

The UNC is an enforcer of the armistice that stopped the fighting in the 1950-53 Korean War and oversees activities inside the DMZ -- a buffer zone between the two Koreas since the three-year conflict ended without a peace treaty.

Jeon Ha-kyou, the defense ministry's spokesperson, said it would "actively support" the UNC's investigation.

The defense ministry said Seoul's recent anti-Pyongyang broadcasts through border loudspeakers do not run counter to the armistice treaty. "It is part of our self-defense right (against the North's recent launches of garbage-carrying balloons)," the ministry said.

The investigation comes amid heightened cross-border tensions set off by the North's recent trash-carrying balloon campaign.

Since May 28, the North is estimated to have launched more than 1,600 balloons in what has been called a "tit-for-tat" response to anti-Pyongyang leafleting by activists in South Korea.

The UNC is also investigating the North's garbage-loaded balloon launches and has called them a violation of the armistice.

For years, North Korean defectors in the South and conservative activists have sent leaflets to the North via balloons to help encourage North Koreans to eventually rise up against the Kim family regime.

North Korea has bristled at the propaganda campaign amid concern that an influx of outside information could pose a threat to its leader Kim Jong-un.

It has also reacted angrily to the South's border loudspeakers that have aired messages critical of the North's regime, firing artillery shots toward the South in 2015 over the broadcasts. (Yonhap)