Investigation is conducted into a Japanese fishing vessel that Russian border guards detained near the South Kuril Islands, the Foreign Ministry said Friday.
The schooner, Eiko Maru 78, was catching fish in the area in accordance with a 1998 agreement between Russia and Japan, when Russian officials stopped it 3.5 nautical miles from the Kunashir Island for checks Thursday, the ministry said.
"Checks revealed products banned for fishing onboard. The ship was detained," the ministry said.
The Japanese authorities protested Friday against the detention, and demanded a release of the five crewmembers.
"We are closely following the situation," Secretary General of the Japanese Cabinet Shinzo Abe told a news conference.
The crew is currently held in a pre-trial detention center on the Kunashir Island.
Kunashir is one of the four islands that Japan calls its "northern territories" and seeks to retrieve as a condition of a peace agreement with Russia.
Osamu Kon, the head of the Japanese Coast Guard, said the schooner had been detained 5.5km (3 miles) from Kunashir.
Under the bilateral agreement, Japanese fishermen are allowed to enter Russian waters to catch specified types of fish at set periods of time. Russian inspectors said the Japanese fishermen were catching banned sea products, Russian information agency report V.A.