South Korean media, Yonhap, with the report that South Korea fired warning shots at a North Korean vessel that crossed the NLL. The Northern Limit Line ( NLL) is a maritime demarcation line in the Yellow Sea between the Democratic People’s Republic of Korea (DPRK) on the north, and the Republic of Korea (ROK) on the south. The border line is, of course, disputed.
UPDATE – North Korea has fired multiple rocket launchers in response.
Pic via NY Times: