The artillery shells were fired from Jangsan Cape in South Hwanghae Province and Kumgang County in Kangwon Province from around 2:59 p.m. local time, South Korea’s Joint Chiefs of Staff said on Monday afternoon.

The artillery shots fell inside the buffer zone, but north of the Northern Limit Line, violating a 2018 deal. During the inter-Korean summit on Sept. 19, 2018, the two Koreas agreed to cease all live-fire and maritime maneuver exercises in the buffer zones in a military tension reduction deal. North Korea has violated the deal 16 times since.

The South Korean military sent several warning messages to North Korea over the artillery firing, the JCS said, adding that the shots were presumed to be fired from multiple rocket launchers.

“The artillery firing toward eastern and western maritime buffer zones is a clear violation of the Sept. 19 military agreement,” the JCS said, strongly urging the North Korean military to immediately stop such acts.

“Our military is reinforcing a readiness posture in preparation for contingencies while monitoring and tracking related North Korean movements in close coordination between South Korea and the United States.”

North Korea’s artillery firing came as South Korea and the US Forces Korea on Monday began conducting two-day live-fire drills in Cheorwon County, Gangwon Province, according to the office of Cheorwon County.

The South Korean and US militaries were set to fire around 50 artillery shells from multiple-launch rocket systems and 140 shells from K-9 self-propelled howitzers.

The North Korean military on Monday said the artillery firing was in response to live-fire drills conducted by the South Korean and US militaries in an English-language statement issued by an unnamed spokesperson for the General Staff of the Korean People's Army.

The KPA General Staff ordered military units on the eastern and western fronts to fire more than 130 “naval live-shells for ‘tit for tat’ warning” from 3 to 4 p.m. local time.

The North Korean military pledged to respond to the enemy’s all provocative actions with “staunch and overwhelming military action.”

“The enemy side would be well advised to immediately stop their military action of escalating tension in the immediate vicinity of the front where it is within eyeshot,” the spokesperson for the KPA General Staff said. “We severely warn the enemy side to be prudent, not kindling the flame of escalation of tension unnecessary in the area around the front.”

In October, North Korea continued to fire hundreds of artillery shells into inter-Korean maritime buffer zones. It justified the barrages as tit-for-tat moves against regular live-fire drills conducted by the South Korean and US militaries near the inter-Korean border.