A North Korean patrol ship crossed the de facto inter-Korean sea border, or Northern Limit Line (NLL), around 1 a.m. Friday.

The ship returned to North Korea after it received a warning from the South Korean Navy.

After leaving a naval base in the Yellow Sea, the North Korean vessel sailed one-point-eight kilometers into South Korean waters without any prior notice.

The South Korean Navy immediately dispatched speedboats to urge the North Korean ship to return to the North Korean side, which it did at 1:10 a.m.

Friday’s infraction was North Korea’s ninth violation of the sea border this year.