The South’s Joint Chiefs of Staff had no other immediate details of the Thursday afternoon launch, which was the second such incident in the past five days.
“We are still analysing whether it is a single or multiple projectiles,” Kim Joon -Rak, a JCS spokesman told AFP news agency.
But analysts have said that if the North returns to the kind of longer-range banned weapons that it tested in 2017, when many feared a Washington-Pyongyang standoff could end in war, it will be a strong sign that a frustrated North Korea is turning away from diplomacy.
Reporting from Seoul, Al Jazeera’s Rob McBride said it was not clear yet if it was a missile.
“The fact that it was launched from the west side of the peninsula suggests that it was more than a long-range artillery or some sort of tactical battlefield system because in theory it would have to clear the Korean Peninsula. It suggests some sort of a missile launch,” he said.
Pyongyang earlier on Thursday described its earlier firing of rocket artillery and an apparent short-range ballistic missile on Saturday as a regular and defensive military exercise, and ridiculed South Korea for criticising the launches.
North Korea and the United States are currently deadlocked in diplomacy meant to rid the North of its nuclear arsenal.
Late on Wednesday, Stephen Biegun, the US Special Representative on North Korea, arrived in Seoul for talks with South Korean officials on the allies’ approach towards Pyongyang.
It is Biegun’s first visit to Seoul since the Hanoi summit between US President Donald Trump and the North’s leader Kim Jong Un collapsed without agreement.
North Korea conducts tests for ‘tactical guided weapon’: report (02:05)
North Korea wants widespread sanctions relief in return for disarmament steps that the United States has apparently seen as insufficient.
Longer-range ballistic missile tests, banned by the United Nations and seen as threatening by surrounding countries, would likely result in more sanctions.
Pyongyang, Seoul and Washington have refrained from calling Saturday’s launch a missile, which could jeopardise the ongoing diplomacy by violating UN Security Council resolutions as well as Kim’s promise of a freeze on long-range missile tests.
The North has said Saturday’s drill involved multiple Pyongyang “long-range multiple rocket launchers and tactical guided weapons”.
But experts say the North launched at least one short-range missile during the exercise, with a report on the 38 North website suggesting that it was a “direct import” of a Russian-produced Iskander.
“The debris generated by the launch in North Korea is a virtual match of a launch of Iskander conducted by Russia,” it said.
If North Korea imported Iskanders from Russia, the report added, “it has an existing capacity to deliver warheads to targets in South Korea with great precision”.
Inside Story: Why did Pyongyang fire new missiles? (25:00)
Al Jazeera and news agencies