North Korea began holding live-fire drills in seven parts of its disputed western maritime border on Monday. Though such drills are relatively common for the northern country, they rarely announce them in advance, making this instance a particularly transparent display of aggression.

According to the AFP, the South Korean forces began returning fire.

It’s not clear if any targets have been hit or casualties recorded, though the area has a history of aggression. In early 2010, a South Korean ship was sunk by what was widely determined to be a North Korean torpedo (North Korea denied involvement). Later that year, four South Koreans were killed by North Korean artillery fire.

UPDATE 1:38 a.m.: According to the Associated Press:

No shells from either side were fired at any land or military installations, an official with South Korea's Joint Chiefs of Staff said. He provided no other details and spoke on condition of anonymity because of office rules.

Kang Myeong-sung, speaking from a shelter on Yeonpyeong island, which is in sight of North Korean territory, said that anxious islanders were huddled together in shelters. Kang said he didn't see any fighter jets, but he could hear the boom of artillery fire.

We'll continue to update this story as new information becomes available.