The Japan Coast Guard on Thursday extended its search to the sea around Russian-controlled, Japan-claimed Kunashiri Island off Hokkaido for 12 people missing after a tour boat sunk in late April, with Moscow permitting the move based on a bilateral accord on maritime accidents.

The Japanese coast guard notified Russia on April 25 of its intention to expand the search to the area near Kunashiri, one of four islands at issue in a territorial row that has prevented the two nations from signing a post-World War II peace treaty. Russia's coast guard agreed to allow Japanese vessels into the area in line with the International Convention on Maritime Search and Rescue.

Photo taken from operator Shiretoko Yuransen's website shows the tourist boat Kazu I. (Kyodo)

Kazu I, operated by Shiretoko Yuransen, was carrying 24 passengers and two crew members when it went missing on April 23 after leaving port in Shari, Hokkaido, for a cruise along the Shiretoko Peninsula despite a bad weather warning.

Of the 26 who were missing, 14 have been found dead in the sea east of the spot where the boat went down.

According to a local Japanese coast guard, the Russian coast guard told the Japanese side that it had spotted someone drifting at sea with a life jacket but was unable to perform a rescue due to bad weather conditions.

The peninsula in the northeast of Hokkaido was designated as a natural World Heritage site in 2005. It is known as a popular destination for spotting drift ice as well as unique plant life and many rare species of animals such as brown bears.