People watch North Korea's launch of short-range ballastic missiles at Seoul Station, May 30. Yonhap

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 (JCS) 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.

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 launches are known to have involved about three to four missiles.

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.

This photo provided by the Gangwon State Fire Headquarters, shows a balloon sent by North Korea in Chuncheon, Gangwon Province, Sept. 7. Yonhap

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.

JCS spokesperson Col. Lee Sung-jun said in a regular briefing that the North may have fired the missiles in protest of recent South Korea-U.S. exercises or they could be performance tests for missiles to be exported to Russia.

The launches also took place as large-scale naval drills between Russia and China kicked off earlier this week.

Hong Min, a senior research fellow at the Korea Institute for National Unification, said the launches could be a separate form of protest from North Korea against the U.S. during the Russia-China drills.

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)