Authorities were on heightened alert Thursday over the possibility of land mines planted by North Korea troops floating into South Korean territory with the heavy rainfall.

There is also a possibility that North Korea could intentionally float land mines toward the South via shared waterways to protest anti-North Korea leaflet-carrying balloons, according to the Joint Chiefs of Staff.

North Korean troops have been newly planting land mines on the northern part of the Demilitarized Zone separating the two Koreas since April. The JCS estimated that the North has planted tens of thousands of new mines in the area, adding to the hundreds of thousands that are already there.

Among the recently planted explosives, leaf-shaped land mines that could easily be mistaken for tree leaves were detected by the South's military, raising concerns about civilian risks and difficulties in identifying them.

Wooden box mines planted by North Korean soldiers have also drifted into South Korea amid heavy rains in the past.

The JCS pointed out that the North tends to plant land mines without safety measures such as for landslide prevention, noting the possibility that land mines flowing down is higher amid the heavy rains "whether intentional or natural."

North Korea also appeared to have "significantly increased" the amount of water released from a dam upstream from the inter-Korean border without giving prior notice to South Korea, according to the Environment Ministry.

Satellite imagery taken at around 3 a.m. Thursday showed that the width of the North's Hwanggang Dam on the Imjin River had widened compared to the image taken at around 10 p.m. the previous day, the ministry explained. Based on another satellite image taken at around 3 p.m., the ministry stated that the amount of water released from the dam appears to have increased significantly.

It added that the water level of the South's northernmost Pilseung Bridge on the Imjin River rose to 2.88 meters as of Thursday at 6:20 a.m., a 2.4-meter increase compared to Wednesday. As of 6:30 p.m., water levels reached up to 3.65 meters due to the influence of heavy rain near Imjin River.

The ministry noted that it had immediately updated the local government and military about the situation after confirming the discharge and activated an emergency response system.

Apart from the land mines, officials are also closely looking at other possible provocations by Pyongyang, such as firing across the border or flying drones, adding that they are making all-out efforts to enhance preparedness against any potential aggression by the North.

Amid growing fears, Defense Minister Shin Won-sik called for "firm readiness" against the North's provocations Wednesday during an emergency meeting of top military commanders held to discuss North Korea's potential provocations.

The remarks come after Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, warned Tuesday that the South would pay a “gruesome and dear price” if it continued to send anti-Pyongyang leaflets to the North.

Meanwhile, the Korean military has requested the United Nations Command to notify North Korea of the risks of the land mines flowing into the South as inter-Korean communication channels have been suspended. However, officials said North Korea has not been responsive to the UNC.