The two Koreas fired warning shots at each near a nautical border after Seoul said a merchant vessel from its neighbor crossed the line, adding to a recent series of provocations along one of the world’s most militarized boundaries.

South Korea’s military said it fired warning shots after the North Korean vessel violated the western sea border at 3:42 a.m. Monday. Pyongyang responded by firing about 10 artillery shells less than two hours later toward a South Korean ship that landed on the north side of the line, South Korea’s Joint Chiefs of Staff said.

No damage was reported to any vessel but the incident comes as North Korea has tested border safety agreements in recent weeks by flying warplanes near South Korean airspace and firing scores of artillery shells off its coasts.