North Korea says the missile it tested Sunday was a “newly developed ballistic rocket capable of carrying a large-size heavy nuclear warhead.”

The claim, which was carried by KCNA, the North’s state-run mouthpiece, has not been independently verified. Yonhap, the South Korean news agency, quoted South Korean military officials as saying more information was needed to determine the missile’s technical details, but that, in Yonhap’s words, North Korea “seems to have yet to master missile technology for atmospheric re-entry,” an important aspect of the type of ballistic missile the North claims to have test-fired.

Even if the missile fired Sunday wasn’t an ICBM, experts say the test represents an important step toward the development of such technology. John Schilling, writing in 38North, a website that focuses on geopolitics on the Korean Peninsula, points out that the test “represents a level of performance never before seen from a North Korean missile.” Indeed, the missile reached a height of 1,242 miles and traveled about 435 miles before landing in the sea west of Japan. The AP adds “had been fired at a normal angle, analysts say, it could have flown much farther — estimates vary between 4,000 and 7,000 kilometers (2,500 and 4,350 miles), the upper number putting Alaska and possibly Hawaii within striking distance.”

Schilling says North Korea

appears to have not only demonstrated an intermediate-range ballistic missile (IRBM) that might enable them to reliably strike the US base at Guam, but more importantly, may represent a substantial advance to developing an intercontinental ballistic missile (ICBM). Given speculation over the past months about the possibility of military action by the Trump administration to prevent Pyongyang from acquiring such a weapons, the possible testing of ICBM subsystems in this low-key manner may be a North Korean hedge against the possibility of such action.

The White House, in its response, called for increased UN sanctions on the North, and cited the fact the missile landed close to Russia, noting, “the President cannot imagine Russia is pleased.” The Trump administration’s policy on North Korea has alternated between threats and conciliation, leading to questions about what the U.S. position on North Korea actually is. Russian President Vladimir Putin on Monday said: “We need to return to dialogue with North Korea and stop scaring it and find ways to resolve these problems peacefully.”

South Korea and Japan, which are the likeliest first targets of any North Korean aggression, called for coordinated international action to defuse the threat. Moon Jae In, the newly elected South Korean president, campaigned on a platform of more talks with North Korea. Pyongyang’s missile test came four days after his victory.