South Korea's military conducted routine live-fire exercises near a disputed maritime border with North Korea on Monday, despite the North's threat of a “merciless” retaliation.

South Korean defense officials later said the two-hour drill, which included artillery, mortars and helicopters, ended without an immediate military response from the North.

Earlier Monday, a North Korean radio broadcast warned inhabitants of five front-line South Korean islands to evacuate to avoid possible retaliatory shelling.

“Once the group of traitors starts a reckless military provocation in those waters, trespassing on the DPRK's (Democratic People's Republic of Korea) inviolable marine demarcation line, and in case just a single column of water is observed in its territorial waters, the Korean People's Army will promptly make merciless retaliatory strikes.”

The South Korean and U.S. navies are holding joint anti-submarine drills in the Yellow Sea this week, ahead of annual large-scale war games next week involving thousands of troops from both countries.

South Korea regularly conducts artillery exercises from front-line islands near the disputed border. One such drill in late 2010 triggered a North Korean artillery bombardment that killed four people on the South Korean island of Yeonpyeong.

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