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North Korean officials responsible for the construction of a 23-storey apartment block which collapsed have reportedly been executed by firing squad.

Hundreds of people were feared dead when the structure fell to the ground, sparking a rare public apology and with Kim Jong Un being described as 'upset' and unable to sleep.

High-ranking party officials, police and intelligence analysts were among the 500 people killed in the incident in Pyongyang that was described as "serious" by the state-run Korean Central News Agency.

Four design and construction engineers were shot by a firing squad, according to Japan's Tokyo Shimbun newspaper, while a military official is believed to have been sent to a prison camp.

Pyongyang's expression of "profound consolation and apology" was the first official news of the disaster, which happened in the Phyongchon district of the North Korean capital.

"The construction of an apartment house was not done properly and officials supervised and controlled it in an irresponsible manner," said the statement from KCNA, which is better known for its strident attacks against South Korea and the United States.

The KCNA statement also said the collapse of the apartment building "claimed casualties" but did not give any indication of how many may have been killed or injured. It said a rescue operation ended on Saturday.

An official from South Korea's unification ministry confirmed on Sunday that a 23-storey apartment building had collapsed in Pyongyang last Tuesday, although he would not say from where the information had been obtained.

The official, who asked not to be identified, said the building was presumed to have held 92 families and that it was common for North Koreans to move into new buildings before construction was completed.

"Hundreds are presumed to be dead, assuming that each family has an average of four members," he said.

A spokeswoman for the unification ministry said it was presumed there were four households on each storey but she also said it was not known exactly how many were in the building at the time.

The KCNA statement said North Korean authorities put emergency measures into place to rescue people from the collapsed building and to treat the injured.

It said that Choe Pu Il, North Korea's Minister of People's Security, had "repented", saying he had failed to supervise the project adequately, "thereby causing an unimaginable accident".

The rare apology from the North came while South Korean President Park Geun-hye's administration faces criticism for its handling of a ferry disaster that killed more than 300 people, many of them schoolchildren, last month.

The North launched vitriolic criticism of Park in the wake of that disaster.

On Saturday night, tens of thousands people attended a candlelight vigil in Seoul for victims of the ferry disaster.

Some marched to the presidential palace demanding Park resign, with police arresting 115 of the protesters.