It was Kim's first message that was made public after he returned home empty-handed following the breakdown of last week's summit with US President Donald Trump in Hanoi.

It also came amid reports that there are signs of North Korea restoring part of the Dongchang-ri missile launch site that it began to dismantle last year.

South Korea's spy agency also said vehicle activity is detected at the Sanum-dong rocket assembly center near Pyongyang, which is known to have produced intercontinental ballistic missiles, including the Hwasong-15.

US think tanks and experts floated the possibility of North Korea firing off a long-range missile or a satellite-carrying long-range rocket at either the repaired Dongchang-ri site or the Sanum-dong center.

Trump said he would be "very, very disappointed" with Kim if the reports are true. He added Thursday that he is "a little disappointed."

It is not clear whether Kim's message may be aimed at easing concerns that North Korea could resume nuclear and missile tests that have been halted since 2018.

In a rare revelation, North Korea's Rodong Sinmun, the country's main newspaper, reported the breakdown of the Trump-Kim summit for the first time on Friday.

The country apparently recognized the fact that it cannot conceal the summit outcome indefinitely as North Koreans in border areas can access outside information relatively easily. (Yonhap)