A military source says a large fleet of North Korean fishing boats are operating near the Northern Limit Line (NLL) around Yeonpyeong Island in the Yellow Sea.

The source said there were over a hundred vessels close to the NLL on Thursday, adding that as many as three hundred vessels were spotted on some days.

The Navy has stepped up vigilance in case of the boats' trespassing into South Korean waters. The North Korean patrol boats are also keeping close watch over the fishing boats.

Meanwhile, President Lee Myung-bak ordered preparedness near the maritime border, during the promotion ceremony of Army Chief of Staff Han Min-koo and other four-star generals.

Defense Minister-designate Kim Tae-young said he will request the North Korean side take voluntary control measures to prevent trespassing.