A South Korean vessel collided with a North Korean ship on the Yellow Sea Monday, resulting in the disapperance of three North Korean crew members.

South Korea’s vessel, the 1,500-ton Hyeonsungho-503, was transporting sand late Monday when it crashed into a small North Korean fishing boat in waters near Haeju, north of the Korean maritime border.

Three of the four North Koreans on board went missing after their five-ton fishing boat sank while one was rescued by the South Korean vessel after the wreck.

The Korea Coast Guard said all ten crew members of the South Korean vessel were safe, adding that they will return home once investigation and search and rescue efforts with North Korean authorities are concluded.

Meanwhile, coast guard officials are searching for the missing North Koreans near the maritime border.