North Korea warned on Sunday that it will respond with a “merciless counterstrike” to the South Korea-U.S. joint drill in the West Sea scheduled for Monday.

The navies of the two allies are set to begin a five-day joint anti-submarine drill on Monday as part of their efforts to improve their ability to counter North Korean submarines.

“If a military provocation occurs, a merciless counterstrike will begin,” North Korean military’s western command said, according to Pyongyang’s Korean Central News Agency. Pyongyang’s military added that the area in question is North Korean waters and it will begin a counterstrike as soon as any activity is detected within their waters.

North Korea’s military, however, warned civilians to leave the South Korean territory in question before the forewarned exercises begin.

“In relation to this, those living or working on or around the five West Sea islands should evacuate to safe areas before 9 a.m. on the 20th when the provocative firing drill begins.”

By Choi He-suk (cheesuk@heraldcorp.com)