Japan formally protested Thursday over South Korean trawlers fishing in waters off disputed Russian-held islands that Tokyo also claims. The new row could further upset already strained relations between South Korea and Japan.

The Japanese government summoned Russian and South Korean diplomats Thursday in an escalating row over South Korean fishing in waters near the Kurils, four tiny Russian-held islands off the northern Japanese island of Hokkaido. Japan claims sovereignty over the islands and the surrounding waters. But Russia has allowed South Korean trawlers to fish in the area.

Tokyo opposes Seoul's agreement with Moscow to fish there. Early this week, Japan and South Korea held talks to resolve the fishing dispute, but failed to reach an agreement. On Wednesday, South Korean boats sailed into the waters of the Kurils.

Japanese Prime Minister Junichiro Koizumi said late Wednesday that he is eager to resolve the issue. He says it is regrettable. He calls it a complex issue, because it involves territory, and he says Tokyo must keep discussing the matter with Seoul and Moscow.

Tokyo has criticized Moscow for entering into a fishing agreement that allows a third country to fish in waters around the islands. It believes the fishing accord will harm its position in negotiations with Russia over the islands' ownership. The former Soviet Union seized the islands from Japan at the end of World War Two.

On Thursday, a high-ranking Japanese foreign ministry official urged Russia to consider the situation, saying that it could negatively affect bilateral peace treaty talks, as well as overall relations. He told the South Korean ambassador that Tokyo wants a sincere response to its demand that the fishing be stopped. In a statement, South Korea argued that its fishing pact with Russia has no political significance, and is in keeping with standard international practice.

The dispute is likely to increase tensions between Tokyo and Seoul. Relations are already weighed down by Japan's decision to approve school textbooks that critics say gloss over Japanese wartime atrocities.

Also troubling Seoul is Japanese Prime Minister Junichiro Koizumi's plans to visit the Yasukuni war shrine later this month. The controversial shrine honors convicted war criminals, and Seoul interprets the visit as an affirmation of Japanese nationalism.