SEOUL (REUTERS, AFP) - A spokesman for North Korea's foreign ministry said on Thursday (Dec 21) Pyongyang is not linked to any cyber attacks, the North's first response since the United States publicly blamed it for a massive worldwide cyber security breach.

"As we have clearly stated on several occasions, we have nothing to do with cyber attack and we do not feel a need to respond, on a case-by-case basis, to such absurd allegations of the US," the spokesman said, according to the North's official KCNA news agency.

The US accusation was a serious political provocation against North Korea that Pyongyang would never tolerate, the spokesman said.

The May cyber attack crippled hospitals, banks and other companies. WannaCry infected some 300,000 computers in 150 nations, encrypting user files and demanding hundreds of dollars from their owners for the keys to get them back.

According to experts, North Korea's cyberwarfare targets have expanded from the political - it was accused of hacking into Sony Pictures Entertainment in 2014 to take revenge for The Interview, a satirical film that mocked North Korean leader Kim Jong Un - to the financial, as it seeks new sources of funding.

A South Korean cryptocurrency exchange shut down on Tuesday after losing 17 per cent of its assets in a hacking - its second cyberattack this year, with the North accused of involvement in the first.

Investigators are probing the possibility that Pyongyang was also behind Tuesday's incident, the Wall Street Journal and Bloomberg News reported.

The North is blamed for a massive US$81 million (S$109 million) cyber-heist from the Bangladesh Central Bank (BCB) in 2016, as well as the theft of US$60 million from Taiwan's Far Eastern International Bank in October.

Pyongyang has angrily denied the accusations - which it described as a "slander" against the authorities - but analysts say the digital footprints left behind suggest otherwise.