Kim told reporters Wednesday that he didn't want to speculate about what specific action Pyongyang might take, but he raised the prospect that it could be a nuclear test, as well as another missile launch. North Korea has conducted more than a dozen missile tests this year, including several ballistic missile launches.

"We are worried that in connection with the upcoming April 15 anniversary that the DPRK may be tempted to take another provocative action. We obviously hope not, but we will be prepared," Kim said, referring to the Democratic People's Republic of Korea, North Korea's official name.

Kim reiterated that the US has publicly and privately delivered the message to North Korea that it's willing to engage diplomatically, but he said the US has yet to receive a response.

North Korea has recently resumed digging tunnels and construction activities at its underground nuclear test site, according to five US officials. Commercially available satellite imagery had shown some indications of activity on the surface at Pyongyang's remote Punggye-ri nuclear test site.

It is not yet clear how soon the regime would be capable of testing a device at the site, as it depends on the pace of the activity, the officials say.

The preparations for a possible underground nuclear test come after North Korea tested its first suspected intercontinental ballistic missile since 2017 last month.

"We remain concerned about the North Koreans -- their attempt to continue to improve their nuclear capability as well as their ballistic missile capability," Pentagon press secretary John Kirby told reporters Tuesday. Kirby declined to be more specific about what he was referring to regarding the regime's nuclear capability.

Officials told CNN that US and allied intelligence agencies assess that digging activities at previously shuttered underground tunnel areas are underway, which would be critical for the resumption of underground nuclear testing.

North Korea has conducted six nuclear tests at the site, which lies north of Pyongyang, the most recent and powerful of which was in September 2017.

There are also indications that North Korea's next ballistic missile test could come as soon as the next few weeks, one official said, though the official would not specify the reasons behind that assessment, and US officials say they believe Kim is likely to resume testing a nuclear weapon.

The US intelligence community estimates North Korea could be ready to conduct a nuclear test this year, according to the annual threat assessment report by the director of national intelligence published last month. The DNI said nuclear testing and long-range missiles tests are "laying the groundwork for an increase in tensions" by the regime.

The Defense Department is considering a package of military responses to North Korea's recent test of an improved intercontinental ballistic missiles that could range from flying bombers or sailing warships in the region to beefing up exercises and training, according to the defense officials. Japan and South Korea are being consulted on a potential decision and could be part of any show of force that is decided upon, the officials say.

North Korea reports the missile had a maximum altitude of 3,905 miles and flew a distance of 681 miles with a flight time of 68 minutes.

The missile test was accompanied with Hollywood-style edited video complete with a soundtrack and footage of Kim.