For the first time in two decades, Japan describes islands it disputes with Russia as "occupied."

Japan described four islands it disputes with Moscow as "illegally occupied by Russia", in the latest version of a diplomatic report released on Friday, using stronger language to describe the territorial dispute and underscoring cold relations between the two sides amid Russia's invasion of Ukraine.

The description in the 2022 Diplomatic Blue Book, an annual report on Japan's foreign policy issued by the Ministry of Foreign Affairs, uses this language for the first time in nearly two decades. Japan, which has struggled to improve relations with Moscow to regain control of the Kuril Islands, which Tokyo calls the Northern Territories, has previously described the dispute on a softer tone.

The ministry said in the report that "the Northern Territories are a group of islands over which Japan has sovereignty and which are an integral part of Japan's territory, but which are currently illegally occupied by Russia."

The dispute over the Russian-controlled islands, which the former Soviet Union seized from Japan at the end of World War II, prevented the two countries from signing a peace treaty officially ending hostilities between them.

The report last used a similar term in 2003, but then softened the language until last year, when it described the dispute as "the greatest concern between Japan and Russia" and noted that "Japan has sovereignty" over the islands.

In another territorial dispute, the ministry said the island, which Japan calls Takeshima, is "illegally occupied" by South Korea, which it calls Dokdo.

South Korea's foreign ministry has protested Japan's "repeated inclusion of unfair claims on Dokdo's sovereignty," describing the island as an inalienable part of South Korea's territory. It said Tokyo's repeated allegations were "in no way conducive to efforts to establish a future-oriented relationship between the two sides".