A North Korean submarine has fired a ballistic missile that flew about 500km towards Japan.
It was a show of improving technological capability for the isolated country that has conducted a series of launches in defiance of UN sanctions.
The missile was fired at around 5:30am on Wednesday (local time) from near the coastal city of Sinpo, where satellite imagery shows a submarine base is located, officials at South Korea's Joint Chiefs of Staff and the Defence Ministry told Reuters.
The projectile reached Japan's air defence identification zone for the first time, Chief Cabinet Secretary Yoshihide Suga told a briefing, referring to an area of control designated by countries to help maintain air security.
The distance of the flight indicated the North's push to develop a submarine-launched missile system was paying off, officials and rocketry experts said.
North Korea's "SLBM (submarine-launched ballistic missile) technology appears to have progressed," a South Korean military official told Reuters.
The US Strategic Command said it had tracked what it believed to be a KN-11 submarine-launched ballistic missile.
"I think it was probably successful," said Jeffrey Lewis of the California-based Middlebury Institute of International Studies.
The launch comes two days after rival South Korea and the United States began annual military exercises in the South that North Korea condemns as a preparation for invasion, and has threatened retaliation.
China's Xinhua news agency said the launch could be seen as a response to the drills.
Beijing is Pyongyang's main ally but has joined past UN Security Council resolutions against the North. It has been angered by what it views as provocative moves by the United States and South Korea, including their July decision to base an advanced anti-missile system in South Korea.
The launch came on the same day that the foreign ministers of China, Japan and South Korea were scheduled to meet in Tokyo.
"This poses a grave threat to Japan's security, and is an unforgivable act that damages regional peace and stability markedly," Japanese Prime Minister Shinzo Abe told reporters, adding that Japan had lodged a stern protest.
North Korea has launched numerous missiles of various types this year, including one this month that landed in or near Japanese-controlled waters.
Joshua Pollack, editor of the US-based Nonproliferation Review, said a claim to having mastered the SLBM technology is as much about prestige as a military breakthrough, a status enjoyed only by six countries including the United States, Russia and China.