Japan's ambassador to Moscow was expected Tuesday to respond to Russia's summons over a row about disputed islands visited by President Vladimir Putin for the first time last week.
Bilateral relations have long been dogged by the Kuril islands, known in Japan as the Northern Territories and seized in 1945 by the Soviet Union which then expelled the Japanese population.
Putin, 73, visited the territories last Thursday, prompting Japan to summon Russia's ambassador in protest.
It was the first trip there by a Russian president since Dmitry Medvedev in 2010, who served as head of state from 2008-12 while Putin was prime minister.
Japanese Prime Minister Sanae Takaichi said the islands "are an inherent part of Japan's territory, both historically and under international law."
Moscow said Monday it had summoned the Japanese envoy, Akira Muto, with Foreign Minister Sergei Lavrov saying that Tokyo had "crossed the line of decency".
Muto did not immediately accede to the summons however.
"We were told either that he was not in Moscow or that he was unwell. We did not quite understand the situation," Lavrov said.
Ministry spokeswoman Maria Zakharova later told state news agency TASS that after Lavrov's comments the ambassador had "suddenly started feeling better" and would attend on Tuesday.
Japanese public broadcaster NHK reported that Muto did not initially respond out of concern that Russia would unilaterally dominate the discussions, making a balanced exchange difficult.
However, Tokyo decided to reply to the summons after internal deliberations, NHK said citing unnamed sources.
Ties between Japan and Russia have worsened since the full-scale conflict in Ukraine began in 2022.
Japan has joined other G7 countries in sanctioning Russia and giving money and non-lethal defense aid to Kyiv, which like Washington recognizes Japanese sovereignty over the disputed islands.
Japan indirectly supports Ukraine's air defense by exporting domestically produced Patriot interceptor missiles to the United States, helping replenish US stockpiles that have been reduced by military aid to Kyiv.
Analysts said that Putin's visit to the Kurils could have been aimed at warning Japan off deepening support to Ukraine, potentially by supplying Patriot missiles directly, and cooperation with NATO.
While Japan has eased restrictions on defense exports in recent years, it remains unable to supply lethal military equipment directly to a country at war, including Ukraine.
