It may have taken the collapse of an apartment block in an exclusive district of the North Korean capital to reveal the Achilles heel of young leader Kim Jong Un's secretive regime.

Last week's accident killed the families of people important enough for North Korea to issue an obsequious and unprecedented public apology in a bid to quell public anger, some analysts said.

The 23-story building in Phyongchon, central Pyongyang, was part of a construction boom driven by Kim that includes apartment blocks, roads, bridges and the Masik Ski Resort that has become synonymous with his policy of finishing projects at lightning speed.