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North Korea has once again test fired a ballistic missile, this time via a submarine, South Korea's military said on Wednesday morning (local time), according to the Reuters news agency.

North Korea test-fired the missile at around 5:30 a.m. local time (4:30 p.m. ET) near the coastal city of Sinpo, where satellite imagery shows a submarine base to be located, an official at South Korea's Joint Chiefs of Staff said.

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.

The test is the latest in a series of similar tests by Pyongyang in recent months. Last month, the North test fired three ballistic missiles. In fact, tensions in the region have soared since Pyongyang carried out its fourth nuclear test in January, claiming it was a hydrogen bomb.

The January test followed by a series of missile launches that analysts said show the North is making progress toward being able to strike the United States mainland.

Tensions on the Korean peninsula were exacerbated by the recent defection of North Korea's deputy ambassador in London to South Korea, an embarrassing setback to the regime of North Korean leader Kim Jong Un, noted Reuters.

Earlier this summer, Washington blacklisted Kim over human rights abuses.

Kim responded by threatening a tough response to what he called the United States' “declaration of war” against Pyongyang.