A South Korean defence ministry official told Xinhua that North Korea test-fired a submarine-launched ballistic missile (SLBM) around 5.30 a.m. (local time) off the eastern coastal town of Sinpo in South Hamgyeong province.
The official said North Korea's SLBM technology appeared to have advanced compared with previous launches as the missile flew about 500 km.
It was the longest flight among any known North Korean test-firings of SLBM. The launch came just two days after Seoul and Washington kicked off their joint annual war games codenamed "Ulchi Freedom Guardian" (UFG).
Pyongyang has considered the drills as a dress rehearsal for northward invasion, which the two allies claimed to be defensive in nature.
The Seoul defence ministry official said the launch was Pyongyang's show of force to escalate military tensions on the Korean peninsula.
The UFG computer-simulated exercises will run through next Friday, mounting the already-heightened tensions in the region due to Seoul and Washington's decision in July to deploy a Terminal High Altitude Area Defence (THAAD) battery in South Korea by the end of next year.
The presidential office Cheong Wa Dae convened a national security council (NSC) meeting two hours after the test-firing, reflecting worries about an earlier-than-expected North Korean deployment of submarines capable of carrying nuclear-tipped ballistic missiles within a couple of years.
The meeting was chaired by Kim Kwan-jin, top security advisor to President Park Geun-hye.
