The firing comes a day after North Korea said it fired more than 130 shells into the sea off its east and west coasts, some of which landed in a buffer zone near the sea border between the two Koreas. Seoul called that a violation of a 2018 inter-Korean agreement to reduce tensions.

South Korean and US troops have been conducting live-fire drills near the border since Monday. The allies say the drills are necessary to deter nuclear-armed North Korea, which tested a record number of missiles this year and has made preparations to resume nuclear testing for the first time since 2017.

Pyongyang has sharply criticized the joint exercises as provocative and proof of hostile policies by Seoul and Washington.