SEOUL (REUTERS, AFP) - Should US President Donald Trump call North Korean leader Kim Jong Un "Rocket Man" again, that would represent a "very dangerous challenge" reminiscent of two years ago, a senior North Korean official warned on Thursday (Dec 5).

Choe Son Hui, who was quoted by state-run KCNA news agency, was referring to a time when tensions between Washington and Pyongyang ran high.

Trump on Tuesday indicated that military action was still possible when he was asked about North Korea on the sidelines of the Nato summit in Britain on Tuesday.

He said he still had confidence in the North Korean leader but noted that Kim "likes sending rockets up". "That's why I call him Rocket Man," Trump told reporters.

"We have the most powerful military we've ever had, and we're by far the most powerful country in the world. And, hopefully, we don't have to use it, but if we do, we'll use it. If we have to, we'll do it," Trump added.

Denuclearisation negotiations between Pyongyang and Washington have been deadlocked since a summit in Hanoi broke up in February, and the renewed threats come as a deadline set by Pyongyang for fresh concessions approaches.

North Korea has demanded the US offer it fresh concessions by the end of the year - ahead of Kim's New Year speech on Jan 1, a key political set-piece in the isolated country.

Pyongyang has also issued a series of increasingly assertive comments in recent weeks.