A North Korean military patrol ship retreated after breaching the disputed inter-Korean western maritime border Tuesday morning, marking the first such case since 2018, the Joint Chiefs of Staff said.

The patrol boat crossed the Northern Limit Line at 9:49 a.m. while it tracked another purportedly stray vessel that violated the NLL and crossed into South Korean waters around 15 minutes earlier, according to South Korean military officials.

The patrol boat traveled about 1 kilometer south of the NLL and stayed in the southern part of the NLL for seven minutes.

But the patrol boat turned back toward North Korean waters three minutes after the South Korean Navy’s Chamsuri-class patrol vessel fired three warning shots with the 40 mm naval gun and sent warning messages.

This is the first time that a North Korean patrol boat has breached the inter-Korean maritime border since the two Koreas signed an inter-Korean comprehensive military agreement in September 2018.

Seoul also sent messages to Pyongyang about the North Korean military ship’s breach of the NLL two times through maritime and western military communication hotlines, the South Korean military said.

The South Korean Navy also seized the other steel vessel, which was carrying seven unarmed persons including six in military suits, and towed the vessel into a port at South Korea’s remote island Baengnyeongdo at 11:42 a.m for a formal investigation.

The seven people stated that they did not have any intention to defect to South Korea during the government‘s initial investigation. They reportedly claimed that they deviated from the planned course and accidentally crossed the NLL while moving cargo for relocation.

If the South Korean government confirms their intention to return to North Korea at the joint investigation, Seoul will repatriate the seven people to the North.