South Korean intelligence has warned that another nuclear test by North Korea is imminent, with analysts suggesting that the leadership in Pyongyang wants to "overshadow" next week's meeting between US President Donald Trump and Xi Jinping, the Chinese president.

South Korean government sources confirmed to the Korea JoongAng Daily that preparations at the North's Punggye-ri nuclear proving grounds would be be completed before the weekend.

That assessment tallies with analysis of satellite images released by the 38 North web site, operated by the US-Korea Institute at Johns Hopkins University.

The images show "a heightened level of activity over the past few days", including ongoing work to pump water out of one of the shafts excavated for previous underground tests and the removal of debris from the tunnel.