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 US 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.

The South's Joint Chiefs of Staff said in a statement that the missile was launched at about 11:30 a.m. Seoul time (0230 GMT) in waters east of the Korean peninsula.

The missile was likely fired from the submarine as planned but appears to have failed in the early stage of flight, the Joint Chiefs of Staff said.

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.

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.

South Korea and the United States said on Friday they would deploy an advanced missile defence system in South Korea to counter the threat from nuclear-armed North Korea, drawing a sharp and swift protest from neighbouring 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.-Reuters