Perfecting reentry is "really a matter of enough trial and error to make that work," Stewart said. "They understand the physics, so it's just a matter of design."

Though analysts and North Korea watchers disagree on the hermit state's progress, most say we will not know with certainty if they can pair a nuclear warhead with a missile unless they actually launch one.

In the early hours of Wednesday morning, North Korea conducted its first missile test since September -- a Hwasong-14 that flew higher than any other previous tests.

While it splashed down in waters between the Korean Peninsula and Japan, it flew as high as 4,500 kilometers (2,800 miles), suggesting Pyongyang has the ability to hit "everywhere in the world," US Defense Secretary James Mattis said after the launch.

Tuesday's news conference itself was highly unusual. Senior South Korean officials rarely speak about North Korea's nuclear program at length and with such candor. The government of South Korean President Moon Jae-in seems to be breaking with past policy in suggesting that recognizing North Korea as a nuclear power could be viewed as a realistic way to set the stage for any future negotiations.

Asked whether the Moon government would be willing to have talks with North Korea without the precondition of denuclearization, unlike the two previous administrations, the minister replied, "We do not think there has been a major change in the government's position ... but if necessary we may need to engage in dialogue with North Korea to bring it to the negotiation table."