US Secretary of State Mike Pompeo appeared to have made a diplomatic faux pas and referred to North Korean leader Kim Jong-un as 'Chairman Un' ahead of a visit to Pyongyang on Wednesday.

Washington's top diplomat said the US was beginning "to put some outlines around the substance of the agenda for the summit between the President (Trump) and Chairman Un".

Mr Pompeo faced a backlash on Twitter due to the mistake, which was revealed in the official transcript of his comments to reporters on board his plane to Asia.

"Somebody really needs to have a word with Secretary of State Pompeo before he meets anybody in North Korea," wrote Twitter user Alistair Coleman.

"He just referred to Kim Jong-un as “Chairman Un”. That’s like, I dunno, calling Winston Churchill “Prime Minister Spencer”.