By Harry Howard for MailOnine & AFP

SEOUL — North Korea has reportedly blown up its joint liaison office with the South after Kim Jong Un’s sister threatened military action against Seoul.

An explosion was heard and smoke seen rising from the North Korean border city of Kaesong on Tuesday, the South’s Yonhap news agency reported.

Citing unspecified sources, Yonhap said the smoke was near the joint industrial complex where the North-South liaison office is located.

The explosion came three days after Kim Yo Jung, the sister of the North Korean leader, repeated an earlier threat by saying Seoul will soon witness the collapse of the ‘useless’ inter-Korean liaison office.

The apparent destruction of the joint office, which was opened in 2018 to help the North and South communicate, appears to ratchet tensions further between the two sides amid stalled nuclear negotiations with the US.

Ms Kim, who is first vice department director of the ruling Workers’ Party’s Central Committee, said on Saturday she would leave it to North Korea’s military leaders to carry out the next step of retaliation against the South.

She said: ‘By exercising my power authorised by the supreme leader, our party and the state, I gave an instruction to the arms of the department in charge of the affairs with enemy to decisively carry out the next action.

‘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.

‘Our army, too, will determine something for cooling down our people’s resentment and surely carry out it, I believe.’

On Tuesday, North Korea’s military also threatened to move back into zones that were demilitarized under inter-Korean peace agreements.

The General Staff of the Korean People’s Army said it is reviewing a ruling party recommendation to advance into unspecified border areas that had been demilitarized under agreements with the South, which would ‘turn the front line into a fortress’.

‘Our army is keeping a close watch on the current situation in which the (North-South) relations are turning worse and worse, and getting itself fully ready for providing a sure military guarantee to any external measures to be taken by the party and government,’ said the KPA’s General Staff.

It said it was studying an ‘action plan for taking measures to make the army advance again into the zones that had been demilitarized under the (North-South) agreement, turn the front line into a fortress and further heighten the military vigilance against (the South),’ according to the statement carried by the North’s official Korean Central News Agency.

While it was not immediately clear what actions North Korea’s military might take against the South, the North has threatened to abandon a bilateral military agreement reached in 2018 to reduce tensions across the border.

The Koreas then committed to jointly take steps to reduce conventional military threats, such as establishing border buffers on ground and sea and no-fly zones.

They also removed some front-line guard posts in a symbolic gesture. The North’s statement possibly implies that it would no longer respect the buffer zones and that the guard posts would be rebuilt.

The North’s military also said it would open unspecified areas near the ground border and its southwestern waters so that North Koreans could send anti-South Korea propaganda leaflets to the South, in an apparent tit-for-tat against North Korean defectors and activists floating anti-Pyongyang leaflets across the border.

This could potentially create security headaches for the South if North Korean military vessels escort North Korean civilian boats as they approach or cross the countries’ disputed western maritime border for leafleting, said Kim Dong-yub, an analyst from Seoul’s Institute for Far Eastern Studies and a former South Korean military official.

Choi Hyun-soo, spokeswoman of South Korea’s Defence Ministry, said the South Korean and US militaries were closely monitoring the North’s military and that the inter-Korean military agreement should be kept.

Recently announced as her brother’s top official on inter-Korean affairs, Kim Yo Jong in recent weeks has repeatedly bashed South Korea over declining bilateral relations and its inability to stop leafleting by defectors and activists.

North Korea in recent months has suspended virtually all cooperation with the South while expressing frustration over the lack of progress in its nuclear negotiations with Washington.

The talks have faltered with the Americans rejecting North Korean demands for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.

The North has also threatened to abandon bilateral peace agreements reached during Kim Jong Un’s three summits with South Korean President Moon Jae-in in 2018, while also expressing frustration over Seoul’s unwillingness to defy US-led international sanctions and restart inter-Korean economic cooperation.

Mr Moon on Monday called on North Korea to stop raising animosities and return to talks, saying that the rivals must not reverse the peace deals. (ia/MailOnline)