The lecturers arrived on the island, called Kunashiri in Japanese, on June 15, planning to stay there until late July to hold Japanese language classes for Russians.

They may return home earlier than scheduled as it is difficult to give lessons without the teaching materials, sources said.

Russia's Interfax news agency reported Friday teachers may leave Kunashiri on Monday.

The four islands, located off the northernmost Japan prefecture of Hokkaido, were seized by the former Soviet Union from Japan at the end of World War II and are claimed by Japan.

The longstanding territorial dispute has prevented Tokyo and Moscow from concluding a peace treaty to formally end their wartime hostilities. The islands are known collectively as the Northern Territories in Japan.

According to the Northern Territories Issue Association, the Japanese independent administrative agency that manages the visa-free exchange activities, dozens of cardboard boxes containing teaching materials were seized by Russian customs authorities in Furukamappu, or Yuzhno-Kurilsk in Russian, on the island of Kunashiri. (Jiji Press)