South Korean officials say the sound of an explosion caused residents of a front-line island to prepare to evacuate, and it was later determined it came from a North Korean artillery drill across the rivals' disputed maritime border.

The false alarm Saturday is indicative of the high anxiety between the Koreas in the wake of the North's recent long-range rocket launch and nuclear test.

An official from South Korea's Defense Ministry says the explosion was heard near the northwest island of Baengnyeong at around 7:20 a.m. He says that the South's military believes North Korea fired a single shell from one of its coastal guns.

An official from the county that governs the island says that residents didn't evacuate although fishing boats returned to their ports.