North Korea's in a good mood these days.

The nation is celebrating its first successful test of a submarine-launched missile.

The country's leader — Kim Jong Un — says the US mainland is now within striking range of his nuclear weapons.

That sounds like a threat.

Joel Wit, a former US nuclear negotiator with North Korea, says he's concerned, but not worried. "Because — despite this success — we’re not within striking range of their nuclear weapons."

The threat to the US mainland does not yet exist; there's no evidence North Korea has yet been able to miniaturize its nuclear weapons to fit into a warhead.

It's also extremely unlikely they could get a submarine within range of the US coast: The new North Korean missile only appears to have a range of about 600 miles. But Wit says there is cause for concern, "because this is just one more step in terms of steady progress that North Korea is making in building nuclear weapons and building missiles to deliver them.”

“I’m concerned, and I think it should be ongoing concern for everyone.” The biggest concern, says Wit, is that North Korea is working to develop a working Intercontinental Ballistic Missile, or ICBM. “If they get that thing working then they will be able to reach the United States, and that, of course, is a serious concern for all of us.”

He does not expect North Korea would use weapons like these aggressively, but he says they would seriously complicate relations. He says, for example, North Korea could threaten other countries in order to secure its objectives.

It could also complicate US thinking when it comes to its assurances to allies like Japan and South Korea to defend them. Up to now, those promises were relatively simple, he says. But if North Korea could threaten the US mainland, then that’s a whole different calculation.

Wit concludes, however, by saying he has some understanding of the North Korean concerns. The United States would like to see a unified Korea that’s democratic, “and if you’re North Korean, that would make you fairly nervous.”

A previous version of this story misstated the US allies in East Asia.