A ballistic missile test would jeopardise the diplomatic talks meant to provide the North with concessions in return for disarmament.
A South Korean analyst said that details in the North's media report indicate it could have been a new type of cruise missile.
Another possible clue: one of the lower-level officials mentioned in the North's report on the test — Pak Jong Chon — is known as an artillery official.
The test comes during an apparent deadlock in nuclear disarmament talks after the failed summit in Hanoi between Kim and President Donald Trump earlier this year.
Some in Seoul worry that the North will turn back to actions seen as provocative by outsiders as a way to force Washington to drop its hardline negotiating stance and grant the North's demand for a removal of crushing international sanctions.
A string of increasingly powerful weapons tests in 2017 and Trump's response of "fire and fury" had many fearing war before the North shifted to diplomacy.
Media reports have suggested Kim may visit Vladivostok, Russia, next week for a summit with President Vladimir Putin.
Trump said last month that he would be "very disappointed" if he saw testing.
As the diplomacy stalls, there have been fresh reports of new activity at a North Korean missile research centre and long-range rocket site where Pyongyang is believed to build missiles targeting the US mainland.
During a speech last week, Kim set the year's end as a deadline for Washington to offer mutually acceptable terms for an agreement to salvage diplomacy.
Kim Dong-yub, an analyst from Seoul's Institute for Far Eastern Studies, said North Korea's descriptions of the test show the weapon is possibly a newly developed cruise missile.
The North's report said the "tactical guided weapon" was successfully tested in a "peculiar mode of guiding flight" and demonstrated the ability to deliver a "powerful warhead".
The analyst said the test could also be intended as a message to the North Korean people and military of a commitment to maintaining a strong level of defence even as it continues talks with Washington over nuclear weapons.
The North said Kim mounted an observation post to learn about and guide the test-fire of the weapon.
This is the first known time Kim has observed the testing of a newly developed weapon system since last November, when North Korean media said he watched the successful test of an unspecified "newly developed ultramodern tactical weapon".
Some observers have been expecting North Korea to orchestrate "low-level provocations", such as artillery or short-range missile tests, to register its anger over the way nuclear negotiations were going.
The White House said it was aware of the report and had no comment. The Pentagon also said it was aware but had no information to provide at this point. South Korea's presidential office said it had no immediate comment. South Korea's Joint Chiefs of Staff said they were analysing the test but did not specifically say what the weapon appeared to be.
A US official familiar with monitoring operations said that neither US Strategic Command nor NORAD observed any weapons test. That rules out tests that go high into the atmosphere, such as a ballistic missile, but does not rule out tests at lower altitudes.
- AP
