Updated at 3:41 p.m. ET

The Pentagon says initial assessments show a missile launched by North Korea was an intercontinental ballistic missile; that would make it the third ICBM tested by North Korea.

It’s also North Korea’s first missile launch since mid-September, when Pyongyang sent a missile over Japan.

The latest missile splashed down in the Sea of Japan within Japan’s exclusive economic zone, the Pentagon says. That means that unlike the previous missile, this one did not pass over Japan. The Pentagon also says the missile posed no threat to U.S. territories.

The South Korean Joint Chiefs Of Staff confirmed that a missile was launched around 3:17 a.m. local time and said the flight is being analyzed, Yonhap reports.

The missile flew nearly 1,000 km (about 600 miles) horizontally, with an altitude of 4,500 km (nearly 2,800 miles), Yonhap says.

In response to the launch, South Korea’s military staged a “precision strike” missile exercise of its own, beginning less than 10 minutes after the North Korean launch began, according to Yonhap.

President Trump was briefed on the situation while the missile was “still in the air,” according to White House Press Secretary Sarah Sanders.

The missile was launched during the middle of the night local time, which is unusual — North Korean missile tests normally are held shortly after dawn, as the Washington Post’s Anna Fifeld reported this past summer.

The New York Times’ Motoko Rich, based in Tokyo, reported on Twitter that Japan’s cell phone alert system did not activate to warn citizens about this missile.

This time, as North Korea launched ballistic missile, no cell phone alerts for citizens in Japan (thank goodness. It was before 4am)

— Motoko Rich (@motokorich) November 28, 2017

North Korea periodically tests missiles as part of its ongoing weapons development programs. There’s no indication that this missile launch — North Korea’s 20th this year, according to analysts — was a response to any particular political or military event.

North Korea’s previous two ICBM tests were conducted in July. The reclusive country has been working on the long-range missiles for years.

In such tests, the missiles travel thousands of kilometers in an arc that is more vertical than horizontal. Researchers can use a missile’s trajectory to calculate its potential range.

Melissa Hanham — senior research associate with the East Asia Nonproliferation Program of the Middlebury Institute — explained to NPR how the analysis works. “Imagine a Super Soaker or a hose that you’re shooting up in the air at a very high angle and then coming down not too far away from you,” she said. “If you were to change the trajectory and instead point it away from you, that water would travel a much farther distance.”

A high, steep trajectory thus reveals a missile that could travel on a shallow, long trajectory — potentially striking the U.S., a capability that North Korea has long sought to have.

“As we sort of sat back and held back from negotiations with North Korea, they were able to ramp up their technological experiments,” Hanham says. “And they’ve not only improved the quality of their existing missiles, but they’ve diversified their forces and now added a range of new missiles, including solid-fuel missiles, very far long-range missiles including [ICBMs]. And they’re working on a nuclear warhead as well.”

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