Some of the shells landed in a buffer zone near the sea border in what Seoul said was a violation of a 2018 inter-Korean agreement designed to reduce tensions.

The South Korean military sent several warning communications to the North over the firing, the ministry of defence said in a statement.

North Korea’s military conducted its firing after detecting dozens of “projectiles” fired in the South near the shared border, state news agency KCNA reported, citing a spokesperson of the General Staff of the Korean People’s Army.

“The enemy should immediately cease military actions that cause escalation of tensions in areas near the front lines where visual surveillance is possible,” the unnamed spokesperson said, warning that the North would respond firmly and with overwhelming military action to any provocation.

“We sternly warn the enemy not to stir up unnecessary escalation of tension along the front lines.”

South Korea and the United States were conducting a joint land-based firing drill near the border in Cheorwon County in the middle of the peninsula on Monday. Their drill will continue on Tuesday.

The allies have stepped up joint military exercises this year, saying they are necessary to deter the nuclear-armed North, which has resumed testing of its long-range intercontinental ballistic missiles for the first time since 2017, and has made preparations to resume nuclear testing as well.

North Korea has criticised the joint drills as evidence of a hostile policy by Washington and Seoul.

The 2018 Comprehensive Military Agreement (CMA) was the most substantive deal to come from the months of meetings between leader Kim Jong Un and then-South Korean President Moon Jae-in.

With those talks long stalled, however, recent drills and shows of force along the fortified border between the Koreas have cast doubts on the future of the measures.

South Korea has accused the North of repeatedly violating the agreement with artillery drills this year, including one day in mid-October when more than 500 rounds were fired into the sea.

(Reporting by Soo-hyang Choi and Josh Smith; Editing by Alison Williams and Raju Gopalakrishnan)