In an unusually overt threat to Japan, North Korea said Tuesday that firing a barrage of missiles a day earlier was training for a strike on U.S. military bases in the country — a type of saturation attack that experts say could leave Japan vulnerable.

Monday’s exercise, which Prime Minister Shinzo Abe characterized as “a new level of threat,” involved artillery units from the North’s “Strategic Force tasked to strike the bases of the U.S. imperialist aggressor forces in Japan in contingency,” the North’s state-run Korean Central News Agency reported Tuesday.

Pyongyang demonstrated its growing military capabilities with the apparent simultaneous launch of four ballistic missiles into the Sea of Japan, three of which fell inside Japan’s exclusive economic zone. Japan’s Defense Ministry said they flew about 1,000 km and reached an altitude of about 260 km.

KCNA reported that the four missiles were fired at the same time, quoting North Korean leader Kim Jong Un as saying that they were “so accurate that they look like acrobatic flying corps in formation.”

The report said Kim had personally supervised and ordered the drill.

It added that Kim had ordered the North’s military “to keep highly alert as required by the grim situation in which an actual war may break out anytime.”

The Korean-language version of the KCNA report said the missile launches demonstrated the North’s readiness to “wipe out” enemy forces with a “merciless nuclear strike.”

Observers said the undisguised threat to U.S. bases in Japan was rare, even for Pyongyang, which routinely serves up colorful invectives.

“North Korea provokes but typically with some room for denial,” said Brad Glosserman, executive director of the Honolulu-based Pacific Forum CSIS think tank.

“This removes that space. This is likely a signal in anticipation of the U.S.-ROK exercises. One of Pyongyang’s most important goals is to deter Japan from getting involved in any way in a Korean Peninsula contingency. This is one way of getting that message across.”

ROK is an acronym for South Korea’s formal name, the Republic of Korea.

Meanwhile, Abe said Tuesday that he held talks with U.S. President Donald Trump over the phone, during which the two leaders agreed that the North’s “threat has entered a new phase.”

A day earlier, Abe told a Diet committee session that the test-firing “clearly shows that North Korea is now a new level of threat.”

Experts believe the apparent simultaneous launch represents a potentially dangerous new chapter in the crisis, beyond the Korean Peninsula.

“North Korea is clearly upping the stakes with this new form of ballistic missile testing — a saturation type of test aimed at stoking fear in the U.S. and its allies,” said J. Berkshire Miller, a Tokyo-based international affairs fellow with the Council on Foreign Relations.

“As Abe noted, this ‘new level of threat’ is both a result of the saturation missile testing and referencing the significant reduction in timing between missile tests,” Miller added.

Japan, despite its deployment of a multilayered missile-defense program in conjunction with the U.S. consisting of sea-based Aegis systems and ground-based Patriot Advanced Capabilities-3 systems, remains vulnerable to saturation attacks.

According to Daniel Pinkston, an East Asia expert at Troy University in Seoul, Japan’s defenses could likely take out some missiles in the event of such an attack, though some could get through.

Pinkston, however, offered the caveat that under that scenario, the North “would focus those salvos on U.S. military bases in Japan” and not civilian sites.

In particular, Pyongyang would likely target bases that would be used in support of the United Nations Command if the armistice that ended the Korean War were to collapse and the peninsula were to return to wartime conditions.

Japan and the U.S. have designated seven existing U.S. bases as being available to support UNC activities, according to the UNC Rear, based at Yokota Air Base in Western Tokyo.

While the North has touted its increasingly advanced missile program, experts analyzing photos released by state media Tuesday believe the missiles launched a day earlier were not longer-range weapons, but rather souped-up versions of a Scud short-range missile. This extended-range (ER) Scud can reach distances of about 1,000 km, putting parts of Japan at risk, according to the country’s 2016 Defense White Paper.

The North also possesses a number of other missiles capable of striking Japan, depending on the target and the launch site. These include Nodongs and others such as the submarine-launched ballistic missile (SLBM) that was successfully tested last August.

“Missiles that put Japan at risk would be the SLBM (and its land-based variant, the Pukkuksong-2), ER Scud, Nodong, Musudan and up,” said David Schmerler, a researcher at the Center for Nonproliferation Studies at the Middlebury Institute of International Studies in California.

But perhaps most disconcerting for Tokyo is the ER Scud, which a 1999 U.S. Defense Intelligence Agency report said has “a range comparable to the Nodong, but (is) cheaper to construct.”

Leaked U.S. diplomatic cables have also described the ER Scud as capable of carrying payloads greater than 500 kg — equivalent to a nuclear-weapon sized payload — to a range of approximately 1,000 km, according to the nonprofit U.S.-based Nuclear Threat Initiative.

The uncertainty spurred by the latest missile threat and the effectiveness of defending against salvo attacks is likely to provide momentum to bolster defense capabilities in Japan — including through Aegis ground-based systems and the Terminal High Altitude Area Defense (THAAD) system.

Washington began to deploy the first elements of the THAAD system in South Korea on Tuesday, U.S. Pacific Command said.

Monday’s test “certainly boosts the case for THAAD or Aegis Ashore or even pre-emptive capabilities, although that last is worrisome on several levels,” the Pacific Forum’s Glosserman said. “The key is to increase uncertainty in North Korean calculations.”

Glosserman, however, noted that while Pyongyang has the ability to launch a barrage of missiles at Japan, such a move would undoubtably spell doom for Kim and his cohorts at the hands of a massive retaliation by U.S.-led forces.

“I think of the North Korean option as ‘one and done,’ ” Glosserman said. “They get one shot at an adversary with a WMD warhead and then success or failure, that is the end of the regime as it currently exists.”

Estimates: Center for Nonproliferation Studies and Department of Defense | CREATED BY CNS / FUNDED BY NTI