SEOUL —  North Korea on Wednesday test-fired a submarine-launched missile 500km towards Japan, which Japan said marked the first time a North Korean sub-launched missile had entered its air defence identification zone.

The flight distance, which was tracked by South Korea’s military Joint Chiefs of Staff, far exceeded any previous SLBM tests, suggesting significant progress in technical prowess.

A proven SLBM capability would take North Korea’s nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

"While there are still a lot of questions about the details, this test certainly seems to have been successful," said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies in California.

"This system is still in development, but North Korea is clearly making progress," Lewis said.

Current UN resolutions prohibit North Korea from any use of ballistic missile technology, but Pyongyang has continued to carry out numerous launches following its fourth nuclear test in January.

South Korea has responded by agreeing to deploy a sophisticated US anti-missile system - known as THAAD.

Trilateral talks

Meanwhile, foreign ministers from Japan, China and South Korea held their first talks in more than a year today just hours after the North Korea missile launch.

The apparently successful launch was likely to top the agenda.

The meeting, the first since March 2015, comes as Tokyo, Seoul and Beijing have struggled to find common ground on how to deal with North Korea.

Japanese foreign minister Fumio Kishida said the launch was "absolutely unacceptable," in his opening remarks, adding that the three countries should closely co-operate and lead the global effort to deal with North Korea.

Chinese Foreign Minister Wang Yi earlier called for calm.

"We hope that (the situation) will not become more tense and complicated," he told reporters in Tokyo ahead of the trilateral talks, Jiji Press reported. —  AFP