[KVOLUTION]

A North Korean patrol boat intruded over the South Korean side of their western sea border Thursday, but retreated after a standoff that lasted nearly an hour, an official here said.

The intrusion took place at 2:47 p.m. about 12 km off the South Korean island of Yeonpyeong, near which deadly naval battles erupted between the divided states in 1999 and 2002, a Joint Chiefs of Staff official said.

The boat returned at 3:38 p.m. after repeated warnings by South Korean naval forces guarding the Northern Limit Line (NLL), the official said, speaking on customary condition of anonymity.

The standoff came amid high tension in the Yellow Sea after North Korea warned last week against the safety of U.S. and South Korean vessels operating near the maritime border.

North Korea says the NLL ― drawn by a U.S. commander at the end of the 1950-53 Korean War that ended in a truce ― should be drawn farther south.

"The North Korean boat reached 1.6 kilometers into the South Korean side," the official said. "It appears the boat trespassed over the border while chasing Chinese fishing boats that were illegally operating."

But the official did not rule out the possibility that North Korea had deliberately sent the vessel to further raise tension in the Yellow Sea.

South Korea radioed a warning message to the boat once before and twice after it trespassed, he said, adding the incident marks the third time North Korea has breached the maritime border this year.