State media indicates that North Korea fired two missiles from a submarine.

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North Korea launched two missiles from a submarine in the waters of the
Sea of Japan Sunday morning local time, according to the state-run news
agency KCNA. The missiles flew for over an hour, traveling roughly 1,500
km (932 miles) per hour and performing figure-of-eight shaped patterns
before "precisely" hitting a target. The Central Military Commission of
the Workers' Party of Korea "expressed satisfaction" after the drill,
KCNA reported. The launch took place three days after North Korea fired
off at least six short-range missiles into the Yellow Sea. North Korea
is conducting its winter training and intelligence authorities in the US
and South Korea are monitoring it.

On Monday, US, South Korean and United Nations Command forces on the
peninsula were expected to begin the 11-day Freedom Shield exercises,
which will integrate elements of 'live exercises' with constructive
simulations. The US and South Korean air forces have been conducting
regular air exercises, with a US B-52 bomber escorted by South Korean
fighter jets as it flew into the South's air defense identification
zone.​