South Korea's energy ministry said on Sunday that documents about a potential plan to build a nuclear power plant in North Korea were meant to suggest an "idea" but this has never been pursued as an official project.

This raised questions over whether South Korea's President Moon Jae-in had sought any nuclear energy programme for North Korea as part of his drive to restart inter-Korean economic cooperation.

Many of the files were dated to May 2018, a month after Moon held his first summit with North Korean leader Kim Jong Un.

Shin Hee-dong, spokesman of South Korea's energy ministry, said the files were "internal documents" that were discussed only among ministry officials after the summit, as an idea to consider in the future when the two Koreas can potentially reopen economic exchanges.

'We've confirmed that those documents were considered internally within the ministry as an idea for inter-Korean energy cooperation,' Shin told a briefing.

'But the idea was never pursued as a government policy, and it is not true that we had sought to clandestinely build a nuclear plant.'

Shin said one of the documents stipulated that the idea entailed "high uncertainty" due to the need for nuclear negotiations between North Korea and the United States.

He declined to elaborate when asked about other details, citing an ongoing prosecution investigation, but expressed regret that the ministry had deleted the files.

The documents were among some 530 that the ministry had deleted to conceal that it had distorted feasibility studies to shut down a reactor in South Korea. Prosecutors last month indicted three officials on charges of violating the Criminal Act by damaging public records.

Some of the files were reportedly titled "A plan to build a nuclear plant in North Korea" and 'Tasks for phased cooperation to establish electricity infrastructure in North Korea.'