The Pyeongchang 2018 Winter Olympics' website went down just before the event's Friday opening ceremony, thanks to a cyber-attack, and stayed down for about 12 hours.

Some networks around the games venues also went down.

Speculation quickly suggested the outages were not accidental and at a Sunday evening news conference, Pyeongchang 2018 spokesperson Sung Baik-you confirmed they were caused by an attack.

"We know the cause of that problem, but that kind of issues occur very frequently during the Olympic Games," he said.

"We have decided with the IOC that we are not going to reveal the source."

International Olympics Committee (IOC) head of communications Mark Adams suggested the problem could be ongoing.

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