When North Korean leader Kim Jong-un announced in his annual New Year's address that his country had reached the "final stages" of developing an intercontinental ballistic missile, President Donald Trump tweeted a defiant message: “It won’t happen!"

It appears North Korea may have taken his tweet as a challenge. Satellite images and intelligence briefings reported by South Korean media over the past two weeks offer grim warning signs: two missiles placed on mobile launchers, improvements at a coastal missile site, and increased activity at the country’s main nuclear reactor.

"North Korea's nuclear and missile threats are no longer [just] potential," Hwang Kyo-ahn, South Korea's acting president, said at a press briefing Monday in Seoul, according to the South Korean news agency Yonhap. “Its nuclear and missile capabilities are developing at an unprecedented rate.”

Analysts say the swirl of activity raises the possibility of the North Korean regime testing an ICBM sooner than expected. Choe Kang Il, deputy director general for North American affairs at North Korea's foreign ministry, recently told NBC News that it was ready "at any time, at any place.”

When Mr. Kim first came to power in 2011, American officials were uncertain what to make of him. But as his plans to accelerate his country’s nuclear program have become clear, concerns have mounted over how to rein in the reclusive, hostile nation before it develops the technology needed to deliver nuclear warheads to San Francisco, or even Washington or New York City.

Siegfried Hecker, a Stanford University professor who has traveled to North Korea and who formerly directed the Los Alamos National Laboratory, the birthplace of the atomic bomb, estimates North Korea already has enough plutonium and highly enriched uranium to build 20 to 25 nuclear weapons.

“The nuclear program has continued to expand dramatically in the past few years – more nuclear materials, more nuclear weapons, and more sophisticated nuclear weapons,” Professor Hecker says in an email. “All of these are in the hands of a young leader about whom we know very little and a military about which we know even less. That is the real risk – it is here today.”

Five nuclear tests

North Korea has conducted five nuclear tests, including two last year, but the country has never successfully launched an ICBM. If such a launch were successful, it would be a major step toward Pyongyang's goal of targeting the continental United States with a nuclear-armed warhead.

How President Trump would respond to an ICBM test has observers and officials in East Asia on edge, given his bellicose rhetoric and willingness to challenge conventional wisdom.

“North Korea has a wonderful habit of greeting US presidents with a bit of fireworks,” says Andrei Lankov, a history professor at Kookmin University in Seoul who grew up in the former Soviet Union and studied at a North Korean university. In 2009, North Korea welcomed the Obama administration with a long-range rocket launch followed by a nuclear test. But Professor Lankov says Trump could be a “game changer” because of his impulsiveness and mercurial approach to foreign policy.

“The option of a military operation hasn’t been taken seriously for decades,” Lankov says. “Now it is being discussed with an intensity I cannot remember.”

Lankov says a preemptive strike on North Korean nuclear instillations – perhaps similar to Israel’s bombing of a Syrian nuclear facility in 2007 – would likely paralyze its program. But the consequences would be enormous, he says, including a potential retaliatory strike on Seoul.

Trump, who has been a sharp critic of the 2015 Iran nuclear deal, has provided few detailed policy proposals for how he plans to approach North Korea. The White House website says his administration will develop a “state-of-the-art missile defense system to protect against missile-based attacks from states like Iran and North Korea,” and his campaign's position paper talked of more "modern destroyers to counter the ballistic missile threat” from both countries. That would appear to indicate continued support for deploying an advanced American missile defense system in South Korea, despite Chinese and Russian objections.

Another major question is how the Trump administration will engage with China in dealing with the North. China has long been reluctant to rein in its rogue neighbor for fear of collapsing the regime and helping reunify the Korean peninsula. But Trump, like Presidents Obama, Bush, and Clinton before him, appears to believe that Beijing, Pyongyang's only political ally and economic lifeline, needs to do more. The challenge his administration faces is how to convince the Chinese to keep the North Koreans in check while at the same time pressuring Beijing on everything from trade to its military buildup in the South China Sea.

Zhao Hai, a research fellow at the National Strategy Institute at Tsinghua University in Beijing, says it will be a difficult, if not impossible, balance to maintain.

“This kind of pressure will not give China confidence that it can work with the US to further curb North Korea’s nuclear ambitions,” Mr. Zhao says, adding that Trump’s unpredictability makes the situation even more precarious. “We should be extremely worried right now.”

Although a military option could be on the table for Trump, many experts say the complexities of the North Korea issue will ultimately lead him to the same playbook used by previous administrations, which have largely relied on sanctions and strained attempts at negotiations.

“You get into office and you realize that we don’t have a whole lot we can risk,” says David Kang, a professor of international relations at the University of Southern California. “I don’t think Trump is going to be any different. His rhetoric may be different, but I don’t think his policies will be.”

Closer than ever

Trump’s policies aside, many experts agree that North Korea is closer than ever before to deploying a nuclear-tipped ballistic missile. Kim has already conducted more ballistic missile tests, including from submarines, in his five years in power than his father did during his entire 18-year reign. The North launched more than 20 last year alone.

Still, North Korea is likely five or more years away from having the ability to reach the US mainland with a nuclear-tipped missile, says Hecker of Stanford. He says the more immediate threat is its ability to reach all of South Korea, Japan, and some US assets in the Pacific, such as Guam.

With the threats mounting, Hecker and other prominent North Korea nuclear experts have called on the Trump administration to directly engage with Pyongyang. During his campaign, Trump said he was willing to meet with Kim to end his country’s nuclear program. Bilateral talks haven’t been attempted since 2002, when George W. Bush’s administration accused the North of violating the 1994 “Agreed Framework.” The agreement – aimed at North Korea freezing and eventually dismantling its nuclear program – collapsed. (Multilateral talks that began in 2003 and included China, Japan, Russia, the US, North Korea, and South Korea, broke down in 2009.)

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Critics, including the highest-ranking defector from North Korea, have warned against compromising with the Kim regime and argue for letting international sanctions take their toll. But Hecker says bilateral talks are the best option.

“President Trump’s immediate challenge is to prevent the use of these weapons,” he says. “The best way is to talk to the North Koreans to defuse a potential nuclear catastrophe. Eliminating the weapons will be a long-range endeavor.”