(SEOUL) — South Korea’s military said Tuesday it detected explosions on two cross-border roads connecting the country to North Korea, amid a significant recent deterioration in inter-Korean relations.

The South Korean Joint Chiefs of Staff said in a statement that North Korean forces detonated explosives on the Gyeongui Line and Donghae Line, “presumably to block connection roads.”

North Korean forces also deployed “heavy equipment” for “further operations,” the JCS said.

There was no damage to South Korean forces, it added.

“Our military has fired warning shots in areas south of the Military Demarcation Line,” the JCS statement said.

“The military is closely monitoring North Korean activities and has reinforced surveillance and alert posture in cooperation with the U.S., maintaining a state of full readiness,” the JCS said.

Seoul warned on Monday that Pyongyang appeared to be preparing to destroy the roadways.

“The North Korean military has been carrying out activities assumed to be linked to explosions on the roads along the Gyeongui and Donghae lines,” JCS spokesperson Col. Lee Sung-jun said.

“If North Korea undertakes a provocation, we will strongly retaliate in terms of our right to self defense,” he added.

The North Korean military said last week that it would permanently seal off its border with its southern neighbor, cutting rail and road connections and reinforcing defensive fortifications.

The Gyeongui line connects the western South Korean city of Paju to North Korea’s city of Kaesong, while the Donghae line runs along the east coast. North Korea has spent months laying mines, removing lights and dismantling buildings along the two routes, Seoul said.

Pyongyang said it communicated its intentions to U.S. forces in South Korea to “prevent any misjudgment and accidental conflict over the fortification project.”

Tuesday’s explosions followed almost a year of rising cross-border tensions, with North Korean leader Kim Jong Un having declared the two nations as “hostile states” in December 2023.

North Korea has sent thousands of “trash balloons” into South Korea since May, according to tallies released by the JCS.

South Korean civic groups also send balloons into North Korean territory, often carrying rice, essential medicine and leaflets critical of the regime in Pyongyang. North Korea has repeatedly protested such action and threatened a response.

Last week, North Korea accused South Korea of flying drones over Pyongyang and ordered eight artillery brigades to be ready to open fire across the border, according to state media.

Seoul’s Defense Ministry warned that Pyongyang will see “the end of its regime” if it causes any harm to South Koreans.

Editor’s note: This story has been updated.

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