South Korea's navy on Friday fired warning shots to chase away two North Korean ships after they briefly crossed a disputed western sea boundary, Seoul defense officials said.

The North Korean ships -- one military vessel and the other a fishing boat -- were in South Korean-controlled waters for less than 10 minutes Friday morning before they retreated, the officials said requesting anonymity citing department rules.

They said a South Korean navy ship fired five rounds of warning shots after broadcasting a warning. There were no reports of injuries and damage to the ships of either side.

This kind of incident isn't unusual, as North Korea doesn't recognize the boundary drawn unilaterally by the American-led U.N. command at the end of the 1950-53 Korean War. In February, South Korea also fired several rounds of warning shots as a North Korean patrol boat moved south of the boundary but no clash occurred either.

The two Koreas fought three bloody naval skirmishes in the area since 1999. Fishing boats from the Koreas also jostle for positions in the waters teeming with crab and other seafood, especially in the April-June season.

Friday's incident happened as North Korea is stepping up pressure on South Korea to accept its calls to resume talks after months of animosities triggered by its fourth nuclear test in January. Seoul has rejected the overture, saying it lacks sincerity and Pyongyang must first demonstrate how serious it's about nuclear disarmament.

Critics say the North often takes conciliatory gestures after raising tension in an attempt to wrest concessions from its rivals.