Russian border security authorities seized five Japanese fishing vessels Tuesday and took them to the island of Kunashiri, one of the disputed Russian-held islands off Hokkaido, the Japanese government said.

The seizure of the boats, which were operating under a bilateral agreement on fishing around the islands, occurred because Russian authorities suspected discrepancies between the amount of fish caught by the boats and the figures recorded in their daily logs, Japanese government officials said.

The seizure of the boats came in the middle of a trip by Foreign Minister Toshimitsu Motegi to Moscow for treaty negotiations with Russian counterpart Sergey Lavrov.

Japan’s Foreign Ministry said it made a complaint to Russia about the seizure and has called for their release from a “humanitarian standpoint.”

The boats belong to several fishery cooperatives in Nemuro, eastern Hokkaido, and were catching octopus when they were stopped and searched by the border authorities. The vessels were taken to a port in Furukamappu, also known as Yuzhno-Kurilsk, for further investigation, Chief Cabinet Secretary Yoshihide Suga said.

According to Hokkaido fishery authorities, 24 people are believed to have been on the vessels. They are believed to be in good health, Suga said.

“We haven’t been able to contact them. It’s worrying,” said a person belonging to one of the fishery cooperatives.

Three other boats that were with them were released and have returned to Nemuro, according to government sources.

The boats were operating in waters around the Habomai islet group, also part of the Russian-held islands called the Northern Territories by Tokyo and the Southern Kurils by Moscow.

Japan’s Fisheries Agency said the vessels were working under a 1998 bilateral agreement that allows Japanese boats to catch octopus, Alaska pollock, Atka mackerel and other species near the islands within certain quotas.

In exchange, the Hokkaido Fisheries Association pays the Russian government ¥21.3 million ($195,000) in “cooperation money” each year for efforts to preserve ocean resources. While the quotas and payment are renegotiated annually, they have remained unchanged in recent years.

Sovereignty over the islands has long been a sticking point in Japan-Russia relations. Tokyo claims the Soviet Union took them illegally soon after Japan’s surrender in World War II, while Moscow maintains the acquisition was legitimate.

The dispute has kept the countries from signing a formal peace treaty for more than 70 years since the war.

While it is not uncommon for Russian authorities to seize Japanese fishing boats — there have been at least eight such cases over the last 20 years — it is very rare for as many as five ships to be held at once.

In late January, a crab fishing vessel from Shimane Prefecture carrying 10 people was detained for one month before being released. In August 2006, a Russian border security ship opened fire at a Japanese crab fishing vessel suspected of poaching near Kunashiri, killing a crew member.