This photo, taken on Wednesday, shows TV news footage on North Korea's firing of ballistic missiles toward the East Sea. (Yonhap)

North Korea launched at least three short-range ballistic missiles into the East Sea on Wednesday, one of which flew across its de facto maritime border with South Korea, according to the South's military.

The Joint Chiefs of Staff said the missile firing from a site in or around the North's eastern coastal city of Wonsan was detected at around 8:51 a.m.

The North's latest provocation came in apparent protest of the five-day large-scale combined air drills of South Korea and the United States that kicked off Monday.

One of the three SRBMs fell into high seas south of the Northern Limit Line. Another one dropped into waters 57 kilometers east of the South's eastern city of Sokcho and the other headed toward Ulleung Island near Dokdo before falling into the international sea, prompting local authorities to issue an air raid alert, it added.

"Our military can never tolerate North Korea's provocative act and will sternly respond to it in close cooperation with the US," the JCS said.

The North's provocation, the second launch in less than a week, came as the allies on Monday kicked off the five-day Vigilant Storm exercise.

The drills involve more than 240 aircraft, including their advanced stealth jets, amid growing speculation that North Korea may conduct a nuclear test soon. (Yonhap)