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 did not immediately report on the artillery fire, but it has been carrying out an increasing number of military activities, including missile launches and drills by warplanes and artillery units that it said were responses to allied exercises in the South.

South Korea and the US 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.

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.

This year North Korea resumed testing its long-range intercontinental ballistic missiles (ICBMs) for the first time since 2017, and South Korea and the US say it has made preparations to resume nuclear testing as well.