North Korea on Monday vowed to "steadily update and strengthen" its nuclear arsenal, rejecting a joint statement by the Group of Seven Foreign Ministers calling for Pyongyang to give up its nuclear ambitions.

U.S. President Donald Trump said on Thursday he still has a good relationship with North Korean leader Kim Jong Un, with whom he held several summits during his first term, and referred to North Korea once again as a "nuclear power.

North Korea has unveiled for the first time what it says is a nuclear-powered submarine under construction, a weapons system that can pose a major security threat to South Korea and the US.