Japan’s defence minister said on Tuesday that cooperation with South Korea and the United States was “critical” to regional security, after US President Donald Trump raised doubts in the Pacific region by downsizing joint military drills with Seoul.

Speaking during a visit to Australia, Defence Minister Shinjiro Koizumi strongly backed security cooperation with key defence partner Washington and neighbouring South Korea.

“As Japan faces the most severe and complicated security environment in the post-war era, the cooperation among Japan, the United States and South Korea is critical to the peace and stability of the region,” Koizumi told journalists.

The Japan-Australia meeting in Canberra came hours after Trump praised his relationship with North Korean leader Kim Jong Un and criticised South Korea for not helping Washington in its conflict with Iran.

Asked about Trump's comments, Australian Defence Minister Richard Marles said Australia was “very concerned about North Korea's nuclear programme as well as its missile programme, and we have consistently made that point”.

“I think in the challenging world that we face... and there are many challenges, but North Korea is the author of quite a few of those challenges,” he added.