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.
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.
This is the longest-range North Korean missile test for a considerable period. It was fired in a high, lofted flight path, reaching an altitude of some 910km. But if fired on a normal trajectory, experts believe it would have had a range of some 1,900km.
It was fired from some kind of underwater platform, possibly a submarine but more likely a specially designed test-rig or barge.
It is a reminder that North Korea is making significant progress with its submarine-launched ballistic missile programme, seen as especially threatening because of the difficulty of finding and tracking boats in the deep ocean.