The Kisuko ship left the Japanese port of Abashiri early on March 18 and sank to the west of the Kunashir Island, the Kurils' southernmost island, late on Wednesday night after hitting ice, a spokesman for the Sakhalin fishing fleet supervision service said.

"According to preliminary data the crew consisted of four people, all of them Russian nationals, two of them were rescued," he said, adding: "Five Russian vessels are searching the area for the other two."

The two crew members have been taken to the port of Golovnino. Their condition is considered as "moderate - serious," the spokesman said adding that one of them was suffering from heart problems.

The ship's captain and chief mechanic are missing, he said, adding that the water temperature in the area, where the boat went missing, is four degrees Celsius (39.2 degrees Fahrenheit) and a person without a wetsuit could only survive under such conditions for about 20 minutes.

The search operation is being hampered by poor weather.