Japan’s ambassador to Moscow has been summoned to Russia’s Foreign Ministry and is expected to appear there today, Moscow announced, after Russian President Vladimir Putin’s visit to the Kuril Islands reignited a territorial dispute between the two countries.

Relations between Russia and Japan have been strained by a dispute over four of the islands in the archipelago, which were seized by the Soviet army in 1945, during the final days of World War II. Japan claims the islands, while Russia considers them part of its territory.

The Russian president made an official visit there last week, prompting an angry reaction from Tokyo, which summoned the Russian ambassador.

Moscow responded by summoning Japanese Ambassador Akira Muto.

Japanese authorities had “gone beyond the bounds of propriety” by criticizing President Vladimir Putin’s trip, Russian Foreign Minister Sergei Lavrov said in a reprimanding tone on Monday.

“We summoned the Japanese ambassador,” he said during a press conference, but Japanese diplomats initially responded that he could not come immediately.

“Either because he was not in Moscow or because he was unwell, we did not quite understand the reason,” Sergei Lavrov added.

Following the Russian foreign minister’s remarks, the ambassador “suddenly felt better and will come to the Foreign Ministry tomorrow,” his spokesperson, Maria Zakharova, later told the state-run news agency TASS.

Sergei Lavrov said he expects Akira Muto to appear today in order to raise the issue of “the conduct of the Japanese authorities” and his diplomatic mission.