Tokyo CNN  — 

Russia has warned Japan of “retaliatory measures” if it expands joint naval exercises with the United States near Russia’s eastern borders.

The threat is just the latest salvo from Moscow, which has been angered by Japan’s support for Ukraine and its growing ties with NATO countries, and is turning up the heat in a long-running dispute over the sovereignty of islands captured by Soviet forces at the end of World War II.

Russian Deputy Foreign Minister Igor Morgulov said Tuesday that US-Japan naval exercises were “potentially offensive in nature,” according to a report on Tuesday by Russian state news agency RIA-Novosti.

“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.”

USS Abraham Lincoln, left, and JS Kongo, front, take part in a US-Japan joint exercise in the Sea of Japan on April 12

However, he did not specify which US-Japan exercises he was talking about – nor did he make explicit what form Russia’s retaliation might take.

Japan has not yet responded to Morgulov’s remarks and has not responded to a CNN request for comment.

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

Earlier in the month, the Abraham Lincoln led similar joint drills in the Sea of Japan, upon which Russia has a lengthy shoreline.

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.”

A lingering World War II dispute

Tensions between Tokyo and Moscow have been increasing, fueled by Japan’s support of Ukraine after Russia’s invasion of its western neighbor and by the dispute between Japan and Russia over the sovereignty of islands north of Japan that were captured by Soviet forces following Japan’s surrender to Allied forces at the end of World War II.

Japan on Friday described four disputed islands as “illegally occupied” by Russia, the first time in two decades it has used such language.

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

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

While that dispute has simmered for decades, Japan’s support for Ukraine has turned up the heat in the relationship between Moscow and Tokyo.

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

Kishida’s announcement came after he spoke with Ukrainian President Volodymyr Zelensky for the fourth time this year.

Earlier this month, Japan expelled eight Russian diplomats and officials due to the war in Ukraine.

Japan’s actions ‘predictable and transparent’

Analysts said Moscow is jumpy right now and taking out frustrations on Japan.

“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.

US Marines take part in a joint amphibious exercise with the Japan Ground Self-Defense Force amphibious rapid deployment unit at a training site near Mount Fuji in central Japan, on March 23.

Drew Thompson, a senior research fellow at the Lee Kuan Yew School of Public Policy at the National University of Singapore, echoed those thoughts, saying increased military cooperation with the US is the sensible thing for Tokyo to do.

“Japan is slowly awakening to the security threats on its periphery, and it’s doing so in a predictable and transparent way consistent with a democracy,” Thompson said.

Cmdr. Hayley Sims, spokesperson for the US 7th Fleet based in Japan, described the joint exercises in the Sea of Japan in early April as “routine bilateral operations.”

“Our training enhances the credibility of conventional deterrence by demonstrating the strength of our bilateral partnerships,” Sims said.

But Russia has a different viewpoint.

“I think it really does show the increased jumpiness of the Russian side, their tendency now to view actions in their vicinity as always being potentially offensive,” Brown said.

He said increased Japanese cooperation with NATO allies including Britain and France, countries with which Russia has disputes in Europe, exacerbate tensions in the Pacific.

“One thing that the Russians really dislike is that Japan in recent years has been strengthening cooperation with other countries beyond the United States,” Brown said.

Russian provocations

Russia has been flexing its military muscles around Japan for the past few years, analysts said.

Satoru Mori, professor of contemporary international politics at Keio University in Japan, said there had been numerous Russian provocations in the past few months, like military drills on the disputed islands