A South Korean military drill under way on the country's northern-most island of Baengnyeong against a presumed North Korean bombardment and violation of the Northern Limit Line, an inter-Korean maritime border in the West Sea.

A US research institute said this week that North Korea may be significantly expanding its nuclear weapons production and could have added six or more weapons to its stockpile in the past 18 months.

The Washington- based Institute for Science and International Security had estimated last year that North Korea had 10 to 16 nuclear weapons at the end of 2014.