Kim Jong Un looks through a submarine periscope
Kim Jong Un looks through a submarine's periscope while inspecting of the Korean People's Army Naval Unit 167. No doubt he had some tips on how to improve the periscope's performance. Korean Central News Agency/Reuters

North Korea has put its elite cyber-warfare unit to work creating malware posing as smartphone games which it posted on South Korean websites and infected up to 20,000 devices.

That is according to a report from the South Korean National Intelligence Service (NIS) which was delivered to the parliament in Seoul this week, alleging that the smartphone games were posted on South Korean websites between 19 May and 16 September.

The NIS said it had worked with website owners to remove the offending games but it didn't reveal what - if any - damage had already been done to those infected with the malware.

The report continues that South Korean government agencies and their affiliates have been hit with up to 75,000 cyber attack since 2010, with many believed to originate in North Korea.

The report is published shortly after the first military talks in three years between North and South Korea ended in stalemate, following the firing of weapons on both sides of the disputed border.

North Korea is widely though to have developed a sophisticated offensive cyber-army with one report in July suggesting it had recently nearly doubled its cyber-warfare unit 5,900 personnel.

The North has always denied any involvement in cyber-attacks against the South, claiming the government in Seoul is fabricating evidence to increase cross-border tensions.