People watch a television news screen showing an explosion of an inter-Korean liaison office in the DPRK's Kaesong Industrial Complex, at a railway station in Seoul on June 16, 2020. (JUNG YEON-JE / AFP)

SEOUL - The Republic of Korea (ROK)'s presidential Blue House on Tuesday expressed strong regret over the Democratic People's Republic of Korea (DPRK)'s demolition of the inter-Korean joint liaison office in the DPRK's border town of Kaesong.

Kim You-geun, first deputy director of the Blue House National Security Office (NSO), told a press briefing that the ROK government expresses strong regret over the DPRK side's unilateral explosion of the inter-Korean joint liaison office building that was opened in accordance to the Panmunjom Declaration.

The liaison office was opened in September 2018 for the round-the-clock communications between the two Koreas in accordance with the Panmunjom Declaration

Kim said the demolition was an act of letting down the expectations of all who wish for the development of inter-Korean relations and the settlement of peace on the Korean Peninsula.

The comment came after the ROK's Unification Ministry said in a statement that the DPRK blew up the Kaesong joint liaison office at 2:49 pm local time (0549 GMT).

Chung Eui-yong, top national security advisor for ROK President Moon Jae-in, convened a National Security Council (NSC) standing committee meeting to discuss countermeasures toward the DPRK's action.

The liaison office was opened in September 2018 for the round-the-clock communications between the two Koreas in accordance with the Panmunjom Declaration.

The declaration was signed by Moon and top DPRK leader Kim Jong-un after their first summit on April 27, 2018 at the inter-Korean truce village of Panmunjom.

The NSO deputy director said all the blame should be placed upon the DPRK for what will happen because of the joint liaison office demolition, warning that if Pyongyang takes action to worsen situations further, Seoul will strongly respond to it.

The joint liaison office has been temporarily closed since January over the COVID-19 outbreak.

According to a military source cited by multiple local media, an explosion was heard and smoke was seen rising up from the Kaesong Industrial Complex, an inter-Korean factory park in Kaesong housing the liaison office.

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The ROK defense ministry said in a statement that its military has been maintaining a firm defense posture while closely monitoring the moves of the DPRK forces around the clock.

The ministry said that the ROK troops will strongly respond to any military provocations from the DPRK, adding that it will try its best to ensure that the military tensions would not be escalated through stable management of situations.

The explosion came after Kim Yo-jong, first vice department director of the Central Committee of the Workers' Party of Korea (WPK) and younger sister of the DPRK leader, said in a statement Saturday night that she had given instructions for decisive action to be taken.

"If I drop a hint of our next plan the South Korean authorities are anxious about, the right to taking the next action against the enemy will be entrusted to the General Staff of our army," she said, indicating a demolition of the liaison office.

The ROK is also referred to as South Korea.

The DPRK has repeatedly lashed out at the ROK in protest of anti-DPRK propaganda leaflets sent by defectors and activists across the inter-Korean border. Pyongyang closed its joint liaison office and cut off all communication lines with the ROK last week.

A general view shows fields and buildings of the the DPRK countryside outside Kaesong, seen across the Demilitarized Zone (DMZ) from the the ROK island of Ganghwa on April 23, 2020. (ED JONES / AFP)

Earlier on Tuesday, the DPRK's state media reported that the army of the DPRK was ready to take action if defector groups push ahead with their campaign to send propaganda leaflets into the DPRK.

Kim You-geun, first deputy director of the Blue House National Security Office, warned that if Pyongyang takes action to worsen situations further, Seoul will strongly respond to it

The General Staff of the Korean People’s Army (KPA) said it has been studying an “action plan” to reenter zones that had been demilitarized under an inter-Korean pact and “turn the front line into a fortress.”

“Our army will rapidly and thoroughly implement any decisions and orders of the Party and government,” the KPA said in a statement carried by the official KCNA news agency.

In response, Seoul’s defense ministry called for Pyongyang to abide by the 2018 agreement, in which both sides’ militaries vowed to cease “all hostile acts” and dismantled a number of structures along the heavily fortified Demilitarized Zone between the two countries.

“We’re taking the situation seriously,” ministry spokeswoman Choi Hyun-soo told a briefing. “Our military is maintaining readiness posture to be able to respond to any situation.”

ROK President Moon offered Monday to have a dialogue with the DPRK to find a breakthrough, saying the time has come that the two Koreas "would no longer sit and wait for improved conditions."

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Moon said he wished to actively find and put into practice the projects that the two Koreas can choose and push for on their own, expressing his anticipation that the DPRK would open its door for dialogue and put ideas together.

Denuclearization talks between the DPRK and the United States have stalled since the second summit between top DPRK leader Kim and US President Donald Trump ended with no agreement in February 2019 at the Vietnamese capital of Hanoi.

The ROK president proposed in April to resume inter-Korean cooperation with the DPRK by jointly tackling the COVID-19 outbreak.