Russia, which has previously expressed his anger over Japan’s support for Ukraine, has now warned the country of “retaliatory measures” if it expands joint naval with the US near Russia’s eastern borders.

Moscow has long raised concerns over the sovereignty of islands captured by Soviet forces at the end of World War II. It has also showed its concerns over Japan’s growing ties with NATO countries.

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On Tuesday, Russian Deputy Foreign Minister Igor Morgulov said that US-Japan naval exercises were “potentially offensive in nature,” Russian state news agency RIA-Novosti quoted.

“We see such actions by the Japanese side as a threat to the security of our country,” Morgulov said. “If such practices expand, Russia will take retaliatory measures in the interests of strengthening its defense capabilities.”

However, it remains unclear which US-Japan exercises he was talking about. Russia also did not elaborate on the type of retaliations it plans if Japan doesn’t end its joint drills with US.

Last week, the US and Japanese navies wrapped up joint exercises in the East China Sea and Philippine Sea headlined by the USS Abraham Lincoln Carrier Strike Group.

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According to the US Navy, the US and Japan routinely hold joint naval exercises in the Indo-Pacific to “maintain stability in a free and open Indo-Pacific region,” CNN reported.

Not only has Japan’s support of Ukraine angered Russia, but its claims over the disputed islands, which Tokyo says have been “illegally occupied” by Russia has also strained the ties between the two countries.

Japan’s Ministry of Foreign Affairs, in its annual diplomatic report released Friday referred to the islands, which Russia calls the Southern Kurils, as Japan’s “Northern Territories.”

Japan sees the islands as “Japanese territories over which Japan holds sovereign rights, but are currently illegally occupied by Russia,” according to reports.

While Russia hasn’t recently reacted to the dispute over the islands, Japan’s support for Ukraine has further tarnished the relationship between Moscow and Tokyo.

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On Tuesday, Japanese Prime Minister Fumio Kishida agreed to provide food and medicine, additional financial support, small drones and protective face masks to Ukraine as the war continues, according to a statement released by the country’s Foreign Ministry.

This week, Kishida spoke with Ukrainian President Volodymyr Zelensky for the fourth time in a year.

Meanwhile, Japan expelled eight Russian diplomats and officials due to the war in Ukraine, which started Feb. 24 and still continues with no agreement being reached between Kyiv and Moscow.

Russia’s latest move of threatening Japan with retaliation has made analysts think that Moscow was overreacting and is jumpy.”

Japan and the US haven’t done anything out of the ordinary … nothing that would really seem to prompt this level of response,” said James D.J. Brown, associate professor of political science at Temple University in Tokyo.