North Korea test-fired two missiles from a submarine on Monday.
This comes in a reaction to South Korea and the United States kicking off their largest joint military drills in five years.
The North said the test verified its nuclear war deterrence, as it slammed the drills between South Korea and the U.S. known as Freedom Shield.
The South Korean military also said it had detected the launch of at least one unidentified missile from a North Korean submarine.
In a statement, the South said; "The Freedom Shield drills with the US involve wartime procedures to repel potential North Korean attacks and conduct a stabilization campaign in the North."
It emphasised that the exercise was a defensive one based on a combined operational plan.
But North Korea views all such exercises as rehearsals for invasion and has repeatedly warned it would take action in response.
The foreign ministry in North Korea also slammed the United States over what it called "the U.S. vicious 'human rights' racket," after the US said it would hold a UN meeting this week on abuses in North Korea.
North Korea is not technically banned from firing cruise missiles under current UN sanctions, although nuclear tests are not allowed.
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