US President Donald Trump says more information about the health of North Korean leader Kim Jong-un will probably be known "in the not too distant future".

Key points: Donald Trump says he knows how Kim Jong-un is, "relatively speaking"

Donald Trump says he knows how Kim Jong-un is, "relatively speaking" Speculation on Mr Kim's health has been in overdrive since he missed birthday commemorations for his grandfather

Speculation on Mr Kim's health has been in overdrive since he missed birthday commemorations for his grandfather South Korean officials have dismissed claims the leader is gravely ill

Speculation about Mr Kim's health began when he failed to attend commemorations for the 108th birthday of his grandfather, North Korea's founder Kim Il-sung.

US media reported he was gravely ill, but South Korea's Unification Minister has since downplayed the speculation, saying there is no evidence of any "unusual developments" in North Korea.

Mr Trump told reporters he was aware of Mr Kim's condition, but did not go into detail.

"I can't tell you exactly," he said during a White House press conference.

"Yes, I do have a very good idea, but I can't talk about it now. I just wish him well.

Donald Trump and Kim Jong-un met in the demilitarised zone between the two Koreas in June 2019. ( AP: Susan Walsh )

"I hope he's fine. I do know how he's doing, relatively speaking," he added.

"We will see. You'll probably be hearing in the not too distant future."

Satellite images appear to show Mr Kim's train parked at his compound in Wonsan. ( Maxar Technologies via AP )

Mr Kim, who is the third member of his family to lead North Korea since it was founded by Kim Il-sung, had never before missed the commemorations for his grandfather's birthday.

There has been no official word from the North on the state of the leader's health, but state media has reported that he recently sent a message of thanks to workers on the country's east coast.

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South Korea's Unification Minister Kim Yeon-chul told a closed-door forum in Seoul that South Korea had "enough intelligence to confidently say that there are no unusual developments" that would back up speculation about Mr Kim's health, his ministry said.

He did not reveal what specific intelligence led to that conclusion, but stressed that it was reached after a thorough analysis.

ABC/AP