SEOUL, Dec. 21 (Xinhua) -- South Korean Navy on Monday expressed deep regret over the Democratic People's Republic of Korea (DPRK)'s claim on the inter-Korean maritime border in the western sea, vowing to defend the existing Northern Limit Line (NLL).

    The remark came after the DPRK said earlier in the day that it was setting a "peacetime firing zone" along the inter-Korean border in the Yellow Sea, warning that its forces may fire on ships crossing into the area.

    In a statement, South Korean Navy described the DPRK's claim as "irrational", saying that Seoul expressed grave concern as Pyongyang's "threat raised unnecessary tension" between the two sides.

    The statement said that the NLL is the de-facto western maritime inter-Korean border, which should be defended, urging to stop all actions that might cause tension in nearby waters.

    South Korean Navy has made full preparations for defending the NLL, the statement said, pledging to sternly respond to any possible "provocation" by the DPRK.

    Earlier in the day, the DPRK's navy spokesman accused South Korea of attempting to turn the NLL into its permanent water border and using military provocation in the region.

    The DPRK rejected the NLL and only recognized the demarcation line it drew in 1999, which was further south of the NLL, the spokesman said.

    The NLL was drawn after the 1950-1953 Korean War by the United Nations Command. South Korea holds the NLL as the de-facto western inter-Korean border, while the DPRK has not accepted it.

    The North demands that the maritime border in the Yellow Sea, which was set closer to its coast should be re-drawn further south, while South Korea has rejected the North's demand.

    The navies of the two sides have had three clashes, in 1999, 2002 and this November, around the NLL waters, which resulted in heavy casualties on both sides.