Wheeling, West Virginia: US President Donald Trump told a cheering crowd at a campaign rally that there was once tough talk "back and forth" between himself and North Korean leader Kim Jong-un "and then we fell in love".

"He wrote me beautiful letters and they're great letters. We fell in love," Trump said at the rally on Saturday (US time).

He joked about criticism he would get from the news media for making a comment some would consider "unpresidential" and for being so positive about the North Korean leader.

"Why has President Trump given up so much?" Trump said in his mock "news anchor" voice. "I didn't give up anything."