SEOUL,  (UPI) — North Korea submarine movement was detected near the western coast of the peninsula, according to South Korean sources.

The unusual activity involved the deployment of several submarines that were detected on South Korean sonar, local news network MBC reported.

The submarines appeared to have been dispatched from an area where Pyongyang keeps its fleet, including the 1,800-ton Romeo-class submersible craft.

The submarines crossed the Northern Limit Line, a disputed maritime border, in an exercise that appeared to be a preparation for a provocation.

South Korea and U.S. intelligence authorities both detected the movements, according to MBC.

A South Korean government official who spoke on the condition of anonymity said two or three submarines “moved south” then briefly emerged from the water.

These maneuvers are all part of regular training exercises and the South is closely monitoring the situation, the official said.

By late Thursday all submarines had returned north of the NLL but in case of an emergency the area is under constant surveillance, the official said.

North Korea operates numerous submarines, including the smaller shark and salmon-class crafts, according to the report.

Kim Dae-young, a senior research fellow at the Korea Defense and Security Forum, said North Korean submarines could launch a torpedo attack against South Korean vessels.

In a time of rising tensions, South Korean firms have been creating core technology to detect incoming submarines.

Daewoo Shipbuilding & Marine Engineering recently completed development of a system to detect underwater radiation noise, Yonhap reported Thursday.

The DURAN Mark-1 detects noise coming from submarines including underwater movements from submarine propellers.

The noise patterns are used to identify the location of the craft, according to the report.