This photo shows the test-firing of a new intermediate-range hypersonic ballistic missile, provided by North Korea's official Korean Central News Agency, Jan. 7. Yonhap

North Korea fired multiple ballistic missiles toward the West Sea on Monday, South Korea's military said, as Seoul and Washington launched their annual military drills, marking the North's first known ballistic missile test since U.S. President Donald Trump's return to the White House.

The Joint Chiefs of Staff (JCS) said it detected the launch from an area near the western county of Hwangju in North Hwanghae Province at around 1:50 p.m., noting that it may have involved close-range ballistic missiles (CRBMs).

CRBMs refer to ballistic missiles that have a relatively short range of less than 300 kilometers.

Under U.N. Security Council Resolutions, North Korea is barred from staging any launches that use ballistic missile technology.

The JCS said it has bolstered surveillance and maintained a full readiness posture under close cooperation with the United States.

South Korea and the U.S. launched their annual Freedom Shield exercise on Monday for an 11-day run.

North Korea has long denounced the allies' joint exercises as a rehearsal for an invasion against it and has a track record of staging weapons tests in response.

The North released back-to-back statements denouncing the latest joint drills, threatening that Seoul and Washington will pay a "horrible" price for their "dangerous provocative act."

Earlier Monday, the North's foreign ministry denounced the allies' exercise as a "dangerous provocative act," saying it will result in an "aggravated security crisis."

South Korea and the U.S. have rejected accusations of invasion rehearsals, describing their joint drills as defensive in nature.

The North last fired multiple short-range ballistic missiles into the East Sea on Jan. 14. (Yonhap)