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North Korea fired a submarine-launched ballistic missile on Saturday but the launch appears to have failed in the early stages of flight, South Korea's military said.

The launch comes a day after the U.S. and South Korea pledged to deploy an anti-missile system to counter threats from Pyongyang, and two days after North Korea warned it was planning its toughest response to what it deemed a "declaration of war" by the United States. That followed Washington's blacklisting of the nation's leader Kim Jong Un for alleged human rights abuses.

A single missile, presumed to be a SLBM, was launched off southeastern shore off Shinpo, Hamkyung Namdo located in northern North Korea at around 11:30 am Seoul time, according to a statement from the South Korea Ministry of National Defense.

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"We can confirm that North Korea succeeded in launching off from the submarine but the ballistic missile failed in the initial part of flying," a military officer from South Korea's Ministry of National Defense told NBC News.

The North has conducted a string of military tests that began in January with its fourth nuclear test and included the launch of a long-range rocket the following month.

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"We strongly condemn North Korea's continued military provocation, which includes its Musudan mid-range ballistic missile launch in last June 22, which is a blatant violation of the UN resolutions," the officer said.

The U.N. Security Council imposed harsh new sanctions on the country in response to North Korea's fourth nuclear test and the long-range rocket.

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"North Korea have achieved a measurable success in mastering technology for mid-range ballistic missiles, that can reach Guam, with nuclear payload through its sixth attempt and likewise, they will continue to test launch SLBM until they get it," Dr. Koh Yuwhan, a professor at North Korean studies at Dongguk University in Seoul, said.

"But, at the same time, North Korea is saying I am going my way regardless of whatever you do as if they are heading towards the endgame," Koh said.

South Korea and the United States said on Friday they would deploy an advanced missile defense system in South Korea to counter the threat from nuclear-armed North Korea, drawing a sharp and swift protest from neighboring China, Pyongyang's sole major ally.

Pyongyang also conducted a test of a submarine-launched ballistic missile (SLBM) in April, calling it a "great success" that provided "one more means for powerful nuclear attack".

A report on 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University in the U.S., said in May that North Korea’s submarine-launched ballistic missile program is making progress, but it appeared that the first ballistic missile submarine and operational missiles are unlikely to become operational before 2020.