A TV screen shows a file image of North Korea's missile launch during a news program at Seoul Railway Station in Seoul, South Korea, on Thursday. (AP)

North Korea fired multiple short-range ballistic missiles toward the East Sea on Thursday, the South Korean military said, a week after Pyongyang warned that South Korea and the United States will have to pay a "dear price" for their joint drills.

"The North Korean missiles flew about 360 kilometers and landed in the East Sea," the Joint Chiefs of Staff said. It said it detected the missiles launched from the Pyongyang area at 7:10 a.m., but it did not provide any further details, such as the number of missiles fired.

The distance, if fired southward, is sufficient to affect major South Korean cities such as Seoul and Daejeon as well as key military facilities in Gyeryong and Gunsan.

The South's military condemned the latest launch as a "provocative act" that gravely threatens the peace and stability of the Korean Peninsula and vowed stern response.

"While closely monitoring North Korea's various activities under a firm combined defense posture between the South Korea and the United States, we will maintain overwhelming capabilities and posture to respond to any provocation," the JCS said.

On July 1, the North fired two ballistic missiles. One of them was short-range, while the other failed and fell inland.

The latest launches came after the North said last week that the South and the U.S. will have to pay a "dear price" for what it called "provocative war" drills after the two countries carried out the joint summertime exercise Ulchi Freedom Shield.

The North has long denounced joint military drills between the South and the U.S. as rehearsals for an invasion and used them as a pretext for provocations. The allies have said their military exercises are defensive in nature.

Thursday's launch also came hours after North Korea sent another batch of suspected balloons containing trash toward the South on Wednesday night, according to the JCS.

Around 20 balloons were floated but they failed to cross the Military Demarcation Line, it added. No balloons were floating in the air as of Thursday morning.

North Korea has sent thousands of balloons carrying trash into the South since late May in retaliation against anti-Pyongyang leaflets sent across the border by North Korean defectors and activists in South Korea.

In response to the balloon launches, the South's military has been blasting daily anti-North Korean propaganda broadcasts through loudspeakers on the border since July 21. (Yonhap)