North Korea said on Monday morning that it had fired two cruise missiles from a submarine earlier on Sunday.
The missile test came a day before South Korea and the United States were due to kick off their largest joint military drills in years.
North Korea's state-run KCNA news agency said the submarine was engaged in underwater drills to demonstrate "the invariable stand" of North Korea to confront a situation in which "the US imperialists and the South Korean puppet forces are getting ever more undisguised in their anti-DPRK military maneuvers."
DPRK stands for the Democratic People's Republic of Korea, North Korea's official name.
South Korea's military initially said it detected the launch of just one missile from a submarine off the port city of Sinpo on North Korea's east coast.
Seoul's Joint Chiefs of Staff said South Korean and US intelligence authorities were analyzing details of the operation.
Freedom Shield drills ramp up
The US and South Korea will on Monday begin 11 days of joint military drills — the largest such exercises in five years, but partly because of scaled-down operations amid the COVID pandemic.
The allies said the drills, known as Freedom Shield 23, will focus on the "changing security environment" that they attribute to North Korea's series of weapons tests in early 2023 and late 2022.
North Korea, meanwhile, often refers to such exercises as a rehearsal for invasion, and seeks to imply its launches are a response to US and South Korean activities.
"It's very regretful that North Korea is using our regular, defensive drills as a pretext for provocation," said Koo Byoung-sam, a spokesperson for South Korea's unification ministry.
"I hope North Korea realises that there is nothing they can earn from escalating tensions on the Korean peninsula."
The test did not come as a surprise to some observers.
"Pyongyang has military capabilities under development it wants to test anyway and likes to use Washington and Seoul's cooperation as an excuse," Leif-Eric Easley, a professor at Ewha University in Seoul, told the AFP news agency earlier on Sunday.