North Korea may have fired a ballistic missile from a submarine, a move that came just hours after Pyongyang said it would resume nuclear talks with the US.
South Korean officials said a missile launched near the port of Wonsan flew about 450km, 280 miles and reached an altitude of 910km, before landing in the Sea of Japan.
If confirmed, this would be a significant escalation from the short-range tests it has conducted since May.
The news sparked swift condemnation.
Japanese Prime Minister Shinzo Abe said it was a violation of United Nations Security Council resolutions which ban North Korea from the use of ballistic missile technology.
Just hours earlier, Pyongyang had said denuclearisation talks with Washington could finally resume later this week.
Negotiations have been stalled since the Hanoi summit between US President Donald Trump and North Korean leader Kim Jong-un in February ended without an agreement.
Experts said the proximity of the test and the talk’s announcement was deliberate.
“North Korea wants to make its negotiating position quite clear before talks even begin,” Harry Kazianis of the Center for the National Interest in Washington DC told reporters.
Pyongyang seems set to push Washington to back off from past demands of full denuclearisation.