ROBERT SIEGEL, HOST:
Tensions have been brewing for a long time over North Korea's growing arsenal of nuclear weapons. But the possibility that they would lead to war now seems much closer. Today in South Korea, Secretary of State Rex Tillerson declared that the U.S. is reassessing its stance toward the North.
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REX TILLERSON: Let me be very clear. The policy of strategic patience has ended. We're exploring a new range of diplomatic, security and economic measures. All options are on the table.
SIEGEL: In a moment, we'll talk with a former U.S. diplomat about what all options might include. But first to NPR's David Welna on the state of the North Korean nuclear threat.
DAVID WELNA, BYLINE: This is how one of the top experts in the United States sizes up North Korea's nuclear program.
SIEGFRIED HECKER: They have a formidable nuclear arsenal.
WELNA: That's Stanford University nuclear scientist Siegfried Hecker. He's been to North Korea seven times. And over the last decade, he's watched that arsenal grow from what he calls a few primitive weapons to as many as two dozen nuclear warheads today. And all this time, he says via Skype, the country has also been developing ever more powerful missiles to deliver those warheads. Hecker says it appears North Korea is not yet able to reach the U.S. mainland with a nuclear-tipped missile.
HECKER: The general guess is that's probably still possibly five years or so away. However, we also believe that North Korea has the ability to reach all of South Korea, all of Japan and possibly some U.S. assets in the Pacific. That is what constitutes the crisis.
WELNA: The latest salvo from North Korea came last month, while Japanese Prime Minister Shinzo Abe was dining with President Trump. It was a simultaneous launch of four missiles aimed in the direction, Pyongyang said, of U.S. bases in nearby Japan. Melissa Hanham is a nuclear arms expert at the Middlebury Institute of International Studies at Monterey. This latest missile test, she says, was a clear message from North Korea.
MELISSA HANHAM: It is quite a scary thought. Now, what they're saying is that they can launch multiple ballistic missiles that can reach Japan, South Korea and carry a nuclear warhead.
WELNA: And the North Koreans, Hanham says, have also made clear they would be ready to use their nuclear weapons immediately should war break out on the Korean Peninsula.
HANHAM: North Korea is making the calculation that it has to use nuclear weapons very early in any standoff. And it becomes very dangerous because they're the use them or lose them kind of sense.
WELNA: What that suggests, says Tom Karako of the Center for Strategic and International Studies, is that the U.S. and its allies in the region should prepare for battle.
TOM KARAKO: We have to be even more ready that deterrence in a crisis could fail. And we have to be ready to end a conflict, terminate a conflict in short order.
WELNA: And the main targets, Karako says, should be North Korea's nuclear weapons sites.
KARAKO: The mission is not so novel. What we're really dealing with is a lot of quantity and very high stakes for missing.
WELNA: But North Korea is now able to use solid fuel in its missiles. And that makes them much more easily moved around without detection. Stanford's Hecker says that makes it virtually impossible to obliterate North Korea's nuclear program.
HECKER: We don't know where those sites are. You simply cannot eliminate the North Korean nuclear threat by bombing them today.
WELNA: The best the U.S. can hope for at this point, Hecker says, is to set back a nuclear arms buildup that's been racing ahead. What's more, he says, a nuclear war cannot be won so it's not worth starting one. Already, North Korea has a battery of non-nuclear warheads aimed at Seoul. A nuclear battle could mean its devastation. David Welna, NPR News, Washington. Transcript provided by NPR, Copyright NPR.