North Korea fired "multiple unidentified ballistic missiles" on Monday, South Korea's military said, the same day Seoul and Washington began a major annual joint military drill known as Freedom Shield.

"Our military has detected at around 4.50am GMT multiple unidentified ballistic missiles fired from Hwanghae province," the Joint Chiefs of Staff said.

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

The United States stations tens of thousands of US soldiers in South Korea, and the allies regularly stage joint drills, which they describe as defensive in nature.

But such exercises infuriate Pyongyang, which regards them as rehearsals for invasion and routinely responds with weapons tests of its own.

Earlier on 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.

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