US President Donald Trump seems to have softened his tone on North Korean leader Kim Jong-un by calling him a "pretty smart cookie" and skirting questions on the threat posed by the hermit regime's missile tests with a cryptic "We'll see what happens" comment.

In CBS' Face The Nation, Trump did, however, say that he would not be "very happy" if Pyongyang moved ahead with another nuclear test.

Without making major claims about Kim, he noted that the North Korean leader had assumed power at a young age and was forced to deal with "obviously very tough people".

When asked by CBS reporter John Dickerson what he thought of Kim, Trump said he had "no idea" whether he was sane or not.

"Obviously, he's a pretty smart cookie," Trump said.

"But we have a situation that we just cannot let what's been going on for a long period of years continue.

"Frankly, this should've been done by the Obama administration. Should've been taken care of by the Bush administration. Should have been taken care of by Clinton."

When pressed on his reaction to another nuclear missile test, Trump said: "I would not be happy. If he does a clear test, I will not be happy. And I can tell you also I don't believe that the president of China, who is a very respected man, will be happy either."

But Trump would not confirm whether his unhappiness would translate to military action. "I don't know. I mean, we'll see," he said.

He would also not be drawn on why so many of Pyongyang's recent tests have ended in failure with the missiles exploding mid-flight.

A better missile delivery system

"I'd rather not discuss it," he said. "Perhaps they're just not very good missiles. But eventually, he'll have good missiles."

He would not confirm if the US had anything to do with the repeated failed ballistic missile tests. "I just don't want people to know what my thinking is," Trump maintained. "Eventually, he will have a better delivery system. We can't allow it to happen."

The Face the Nation interview came after Pyongyang's second failed ballistic missile test in two weeks. On Saturday (29 April) the test missile exploded shortly after being launched from a silo in Pukchang but North Korea put out a typical bullish statement that it would still target the west coast of the US.