SEOUL, South Korea — U.S. President-elect Donald Trump took to Twitter to vow that North Korea won’t develop a nuclear weapon capable of reaching parts of the United States. But it might already have done so.

Views vary, sometimes wildly, on the exact state of North Korea’s closely guarded nuclear and missile programs, but after five atomic test explosions and a rising number of ballistic missile test launches, some experts believe North Korea can arm short- and midrange missiles with atomic warheads.

Trump’s tweet on Monday night U.S. time was in response to North Korean leader Kim Jong Un, who said Sunday in his annual New Year’s address that preparations for launching an intercontinental ballistic missile have “reached the final stage.” He did not explicitly say a test was imminent.

Trump tweeted, “North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!”

North Korea, poor, suspicious of outsiders and governed by a third-generation dictator, is used to being underestimated and mocked. Few believed it could build a nuclear program that would keep U.S. presidents since the early 1990s up at night.

Armed to the teeth, acutely bellicose and not afraid to push tensions on the Korean peninsula to the brink, Pyongyang could be among Trump’s top foreign policy challenges.

Here’s a look at how close North Korea may already be to proving Trump’s tweet wrong:

THE NUKES

There’s a general consensus that Pyongyang has made significant nuclear and missile progress under Kim, who took over after his father, Kim Jong Il, died in late 2011.

Propaganda out of Pyongyang makes clear that North Korea views nuclear weapons as essential to keeping at bay U.S. and South Korean forces it says are intent on its destruction.

Some U.S. experts believe North Korea may have enough fuel for about 20 bombs and can add a possible half dozen more each year.

Fuel is one thing; it’s much more difficult to develop the technology needed to build bombs small enough to fit on missile tips.

THE MISSILES

Outsiders don’t know for sure whether North Korea can arm any of its ballistic missiles, regardless of range, with nuclear warheads yet.

But Siegfried Hecker, a leading North Korea nuclear expert, wrote after last year’s September nuclear test that outsiders should now assume that Pyongyang has “designed and demonstrated” atomic warheads that can be placed on short- and possibly medium-range missiles.

PUTTING IT TOGETHER

Even if North Korea can fit a nuclear weapon on a missile, it has yet to meet the even greater challenge of building a nuclear-tipped ICBM capable of hitting the U.S. mainland.

WHAT’S NEXT

Pyongyang has a habit of taking a swing at new U.S. presidents, so Trump may not have long to wait before getting a fresh look at North Korean nuclear or missile technology.