A new open-source intelligence analysis of North Korean state-run media by missile experts has shown what appears to be the hypothetical target of the country’s test-launches earlier this week: U.S. Marine Corps Air Station Iwakuni in Yamaguchi Prefecture.

On Monday, the North test-fired what experts said were likely four extended-range (ER) Scud missiles, with the official Korean Central News Agency issuing an overt claim that the drill was a rehearsal for striking U.S. military bases in Japan.

Using images released by North Korean state media — including one showing a map detailing the range of the missiles — David Schmerler and Jeffrey Lewis of the Center for Nonproliferation Studies at the Middlebury Institute of International Studies in California, determined that the drill intended to simulate a nuclear attack on the base at Iwakuni.

“This is the first time that the North Koreans have been specific about attacking U.S. Forces in Japan,” Lewis told The Japan Times. “But last year, a North Korean missile unit launched a Nodong to simulate a nuclear attack on Busan, (South Korea).”

Last July, the North said it held a similar drill that “was conducted by limiting the firing range under the simulated conditions of making pre-emptive strikes at ports and airfields in the operational theater in South Korea, ” KCNA said in a dispatch at the time.

An accompanying photo, similar to the one released Tuesday, showed a map that displayed the possible flight path of the missiles from Hwangju, North Korea, to areas near South Korea’s southern port cities of Ulsan and Busan.

Because the flight path of the four missiles launched Monday was about 1,000 km into the Sea of Japan off the coasts of Aomori and Akita prefectures, Schmerler said he initially believed the simulation might be targeting the U.S. air base in Misawa, Aomori.

“But the range would be a push for the ER Scud to be reasonably used,” he said.

Instead, Lewis suggested that U.S. bases in both Sasebo, Nagasaki Prefecture, and Iwakuni would be within range from ER Scuds deployed near the North’s Sohae Satellite Launch Station in Dongchang-ri, where Monday’s missiles were launched.

After discovering the map photo from Monday’s launch, Schmerler compared it with the similar shot from the Busan drill in July, concluding that Iwakuni was the “hypothetical target for the (recent) drill,” he said.

Besides American personnel, the U.S. base at Iwakuni is also home to Fleet Air Wing 31 of the Maritime Self-Defense Force, and other units of the MSDF. At present, the station has about 15,000 personnel, including Japanese national employees. The base is also now home to a squadron of F-35B stealth fighters, after the U.S. deployed them in January, marking the first operational overseas deployment for the high-tech aircraft.

Lewis said Monday’s drill “demonstrates that North Korea’s war plan is to engage in the large-scale use of nuclear weapons against U.S. forces in the region to ‘repel’ an invasion.”

Last week, South Korean and U.S. troops began the first annual large-scale joint military exercise to test their defense readiness against the threat from North Korea, which regularly characterizes the drills as preparation for potential invasion.

Media reports have said the F-35B would participate in the exercises, known as Foal Eagle.

“The addition of the F-35B is meant to deliver a strong message to the North that they could be used against the rogue state in case of a conflict breaking out on the Korean Peninsula,” the Yonhap news agency quoted an unidentified military official as saying last week.

This, said both Lewis and Schmerler, was likely the North’s rationale for targeting Iwakuni.

“North Korea sees Foal Eagle as a dress rehearsal for an invasion,” Lewis said. “So the missile launch is their rehearsal for using nuclear weapons to stop the invasion.”

Asked about the analysis at a daily news conference on Wednesday, Japanese Chief Cabinet Secretary Yoshihide Suga said he was not aware that Iwakuni may have been the simulation’s target, but noted the 1,000-km range would put it and other areas in Japan within the North’s sights.

“The missiles flew for 1,000 km, so if you take that into account, western Japan, including Shikoku, could surely be a target,” Suga said, reiterating Prime Minister Shinzo Abe’s comment that the government views the four missiles’ simultaneous launch as “a new level of threat.”

A spokesman for U.S. Forces Japan also declined comment on the apparent threat to Iwakuni, but reiterated the U.S. commitment to the defense of Japan and South Korea.

“We remain prepared and will continue to take steps to increase our readiness to defend ourselves and our allies from attack, and are prepared to use the full range of capabilities at our disposal against this growing threat,” U.S. Air Force Maj. John Severns said in an email.

Asia security experts said targeting Iwakuni would be an upping of the ante by the North.

“This seems to be an attempt to illustrate capabilities and to test the waters with China and the U.S. during difficult times for them both,” Nick Bisley, a professor of international relations at La Trobe University in Australia said, noting that China is hosting its annual rubber-stamp parliament and the Trump administration is “clearly still getting their feet under the desk.”

According to Euan Graham, director of the International Security Program at the Lowy Institute in Australia, targeting Iwakuni could be a “way of letting us know they have the ability to hit U.S. bases in Japan that are likely to be used in a Korean contingency.”

Graham also noted the implications of the threat to Japanese living nearby.

“I wonder how the people of Hiroshima will feel at the prospect they may again be targeted by nuclear weapons in wartime, given the proximity of the marine base at Iwakuni,” he said.