South Korea's Navy on Friday fired warning shots at fishing boats from the Democratic People's Republic of Korea (DPRK) for violating a tense western maritime border, officials?in Seoul?said.

The six fishing boats, which crossed the border in succession starting from 11:44 a.m. local time, retreated north of the border after the Navy fired two warning shots at 3 p.m., said South Korea 's Joint Chiefs of Staff.

In recent weeks, an increasing number of DPRK fishing boats have crossed the western border called the Northern Limit Line ( NLL), a scene of deadly naval clashes between the estranged neighbors.

The DPRK has refused to acknowledge the NLL, drawn unilaterally by the U.S.-led United Nations Command at the end of the 1950-53 Korean War.

The Navy has reportedly raised the alert level along the sea border in response.