North Korea has blown up a liaison office with the South in the border city of Kaesong, according to South Korea's Unification ministry.

"North Korea blows up Kaesong Liaison Office at 14:49", the office of the spokesman for the ministry, which handles inter-Korean relations, said in a one-line alert sent to reporters.

The statement came minutes after an explosion was heard and smoke seen rising from the long-closed joint industrial zone in Kaesong where the liaison office was located, Yonhap news agency reported citing unspecified sources.

Its destruction came after Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, said at the weekend: "Before long, a tragic scene of the useless north-south joint liaison office completely collapsed would be seen."

Since early June, North Korea has issued a series of strong condemnations of the South over activists sending anti-North Korea leaflets over the border - something defectors do on a regular basis.

Smoke rises from the inter-Korean industrial complex

Last week it announced it was severing all official communication links with South Korea.

The leaflets, usually attached to hot air balloons or floated in bottles, criticise North Korean leader Kim Jong-un for human rights abuses and his nuclear ambitions.

The liaison office was opened in September 2018, days before the South's President Moon Jae-in flew to Pyongyang for his third summit with Mr Kim.

Officials from both sides were stationed at the office during subsequent months, but inter-Korean relations soured following the collapse of the Hanoi summit between Mr Kim and US President Donald Trump in February last year.

Its operations were suspended in January because of the coronavirus pandemic.

Analysts say North Korea may be seeking to manufacture a crisis to increase pressure on South Korea while nuclear negotiations with the United States are at a standstill. 

South Korean soldiers patrol the boundary along the Demilitarised Zone with the North

Earlier today, North Korea's army said it was "fully ready" to take action against the South, included re-entering areas that had been demilitarised under an inter-Korean agreement.

The General Staff of the Korean People's Army (KPA) said it has been studying an "action plan" to re-enter zones that had been demilitarised under a 2018 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.

President Moon urged the North yesterday to keep peace agreements reached by the two leaders and return to dialogue.

The two Koreas remain technically at war after Korean War hostilities ended with an armistice in 1953 that was never replaced with a peace treaty.