South Korea fired shells at North Korean waters on Monday in response to Pyongyang’s holding of live-fire drills near the two adversaries’ sea border.

“Some of the shells fired by North Korea dropped in our area and our side responded with fire,” a South Korean military spokesman told Agence France-Presse.

The news agency also reported that inhabitants of a nearby South Korean island were taken to shelters.

Kim Jong Un’s regime has intensified its belligerent rhetoric and has been holding a series of ballistic missile and rocket tests in response to ongoing annual joint military exercises conducted by South Korea and the U.S.

While Washington and Seoul say their maneuvers are routine and defensive in nature, the North deems them a rehearsal for invasion. The escalation comes amid Sunday’s threat from Pyongyang to conduct a fourth nuclear test.

Write to Charlie Campbell at charlie.campbell@time.com.