“We once again sternly warned (the) enemies that staged maritime provocations on top of the firing of artillery shots and cross-border loudspeaker broadcasts,” the spokesman added, according to South Korean news agency Yonhap.
The maritime border is considered a flashpoint, and has been the location of several clashes between the two Koreas over the years.
Tensions have soared in recent weeks with North Korea conducting several missile launches and artillery barrages that have provoked South Korea and Japan and worried their Western allies.
Pyongyang has dramatically ramped up its military exercises recently as Seoul and Washington say North Korean leader Kim Jong Un is close to conducting what would be his country’s seventh nuclear test.
Monday’s exchange of warning fire comes on the same day United States Deputy Secretary of State Wendy Sherman visits Japan and holds three-way talks with Japan and South Korea, both allies, in a show of unity after North Korea’s slew of launches.