South Korea’s military reported on Tuesday that it had detected explosions on two cross-border roads linking the country to North Korea, signaling a further deterioration in inter-Korean relations.

The South Korean Joint Chiefs of Staff (JCS) stated that North Korean forces detonated explosives on the Gyeongui Line and Donghae Line, likely to block the connection roads. Additionally, North Korean forces deployed “heavy equipment” for potential further operations, according to the JCS.

No damage was reported to South Korean forces, the statement added.

In response, the South Korean military conducted retaliatory fire in areas south of the Military Demarcation Line. “Our military is closely monitoring North Korean activities and has enhanced surveillance and alert readiness in cooperation with the U.S., maintaining full preparedness,” the JCS said.

Seoul had previously warned that North Korea appeared to be preparing to destroy these roadways. “The North Korean military has been conducting activities likely related to explosions along the Gyeongui and Donghae lines,” JCS spokesperson Col. Lee Sung-jun noted. “If North Korea engages in any provocation, we will strongly retaliate in accordance with our right to self-defense.”

Last week, North Korea announced its plans to permanently seal its border with South Korea, cutting rail and road connections while reinforcing its defensive measures. The Gyeongui Line connects South Korea’s western city of Paju to North Korea’s Kaesong, while the Donghae Line runs along the east coast. South Korean officials reported that North Korea has been laying mines, removing lights, and dismantling buildings along these routes for months.

Pyongyang stated that it had communicated its intentions to U.S. forces in South Korea to prevent any “misjudgment or accidental conflict” during the fortification process.