A South Korean naval ship fired warning shots Tuesday to chase off a North Korean patrol boat which crossed their disputed western sea border.

A South Korean naval ship fired warning shots Tuesday to chase off a North Korean patrol boat which crossed their disputed western sea border, military officials said.

The incursion came at 10:20 am local time (01:20 GMT) as military tensions run high across the Yellow Sea maritime boundary, a scene of sporadic battles between South and North Korean vessels, the South's defense ministry said.

"The North Korean boat sailed into the southern side of the sea boundary but retreated after our naval ship fired warning shots," a ministry spokesman told AFP, adding details would be disclosed later.

It was not clear whether the incursion was intentional. North Korean patrol ships sometimes sail into southern waters to track Chinese vessels fishing illegally near the sea border.

Cross-border tensions have remained high this year due to a series of North Korean ballistic missile tests, nuclear threats and annual US-South Korean military exercises.

Pyongyang sees the joint drills as a rehearsal for invasion. South Korea and the US say they are purely defensive.