Pyongyang has dramatically ramped up missile launches and military exercises in recent weeks, as Seoul and Washington say North Korean leader Kim Jong Un is close to conducting what would be his country's seventh nuclear test.

Roughly 250 rounds were launched late Tuesday, South Korea's Joint Chiefs of Staff (JCS) said, calling it a "clear violation" of the 2018 agreement.

"We strongly urge North Korea to immediately halt its actions," the JCS said in a statement.

"North Korea's continued provocations are actions that undermine peace and stability of the Korean Peninsula and the international community," it added.

Pyongyang on Wednesday said the barrage was intended to counter the "enemy's war drill against the North" along the border.

South Korea's military "fired dozens of shells of multiple rocket launchers in the forefront area... from 9:55 to 17:22 on Oct. 18," a spokesman for the General Staff of the Korean People's Army said in a statement carried by the official KCNA news agency.

"KPA units on the east and west fronts conducted a threatening, warning fire toward the east and west seas on the night of Oct. 18, as a powerful military countermeasure."

The North also fired artillery rounds into the military buffer zones last week.