South Korea held a routine military exercise near a disputed border with North Korea Monday despite the northern threat of retaliation if the border is violated.

The drill included cannons, mortars, and helicopters.

An announcer on a radio broadcast from the North warned of an immediate and merciless counter-attack if even a “single column of water” is monitored in North Korean waters during the drill.

No action from the North has been reported.

South Korea regularly conducts artillery exercises from front-line islands near the disputed border off the west coast of the Korean Peninsula. One drill in November 2010 triggered a North Korean artillery bombardment that killed four South Koreans on the island of Yeonpyeong.

U.S. military officers tell VOA that threats of retaliation against South Korean military drills are common and no cause for alarm.