US President Donald Trump said he has a "very good idea" about the status of Kim Jong Un but he can't talk about it now, amid rumours about the ill health of the North Korean leader.

"I can't tell you exactly - yes, I do have a very good idea but I can't talk about it now. I just wish him well," Trump told reporters at a White House news conference on Monday.

There have been unconfirmed reports that Kim is not in good health. The rumours began after he missed the April 15 commemoration of the 108th birthday of his grandfather, North Korea founder Kim Il Sung. North Korea has not dispelled the rumours of ill health of its ruler.

"I have had a very good relationship with Kim Jong Un. If I weren't president you would be... You would have been in war with Korea. You would have been in war with North Korea if I wasn't president that I can tell you. He expected that. That I can tell you," he said.

"I hope he's fine. I do know how he's doing relatively speaking. We will see You will probably be hearing in the not-too-distant future," Trump, who met Kim at least twice to persuade him to give up his nuclear weapons, said.

Reports cited satellite images to say that a train likely belonging to Kim has been parked at his compound on the country's east coast for a week.