WASHINGTON -- US President Donald Trump warned Sunday that he "will not be happy" if North Korea conducts an additional nuclear test, and vowed not to let the communist nation have a "better delivery system."

"If he does a nuclear 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," Trump said in an interview with CBS "Face the Nation," referring to North Korean leader Kim Jong-un.

Asked if his unhappiness would mean military action, Trump only said, "We'll see."

Referring to the North's pursuit of an intercontinental ballistic missile capable of hitting the continental US, Trump said that the North's leader is trying to create a "better delivery system," but "we (the US) can't allow it to happen."

Trump also called the North's leader "a pretty smart cookie."

"He was a young man of 26 or 27 when he took over from his father, when his father died. He's dealing with obviously very tough people, in particular the generals and others," Trump said of the 2011 death of late North Korean leader Kim Jong-il.

"And at a very young age, he was able to assume power. A lot of people, I'm sure, tried to take that power away, whether it was his uncle or anybody else. And he was able to do it. So obviously, he's a pretty smart cookie," Trump said. "But we have a situation that we cannot let what's been going on for a long period of years continue." (Yonhap)