Pyongyang, North Korea - North Korea fired "multiple unidentified ballistic missiles" on Monday, South Korea's military said, the same day Seoul and Washington began their major annual joint military drill.

"Our military has detected at around 13:50 (01:50 AM ET) multiple unidentified ballistic missiles fired from Hwanghae province into the West Sea," the Joint Chiefs of Staff said, referring to the body of water also known as the Yellow Sea.

"Our military will bolster surveillance and maintain a full readiness posture under close cooperation with the United States," the JCS added.

The US stations tens of thousands of soldiers in South Korea, and the allies regularly stage joint drills, which Pyongyang finds threatening.

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Earlier Monday, the nuclear-armed North slammed the drills as a "provocative act," warning of the danger of sparking war with "an accidental single shot."

"This is a dangerous provocative act of leading the acute situation on the Korean peninsula, which may spark off a physical conflict between the two sides by means of an accidental single shot," said Pyongyang's foreign ministry, according to the Korean Central News Agency.

Last week, Pyongyang slammed the US for "political and military provocations" over the visit of a US Navy aircraft carrier to the South Korean port of Busan.

The joint US-South Korea Freedom Shield 2025 exercise kicked off on Monday, and will involve "live, virtual, and field-based training" and will run until March 20, according to a US statement.