Japan’s defense ministry reported it had tracked four large Russian ships transporting troops through a strait north of the country and heading west. 

Officials speculated that the vessels could have charted a course for Europe based on their trajectory. Two ships passed through at around 8 p.m. Tuesday and another two at around 7 a.m. Wednesday. 

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The Defense Ministry published photos that depicted a vessel identified as one typically used to land expeditionary forces for amphibious assaults. One vessel appeared to have military trucks loaded onto its deck, The Mainichi reported. 

Japan Prime Minister Kishida

Japan's Prime Minister Fumio Kishida answers reporters' questions about a measure against the new omicron variant of the coronavirus, at his official residence in Tokyo on Monday.  (Sadayuki Goto/Kyodo News via AP)

A Defense Ministry spokesman said "it is possible" the four ships could be headed for Ukraine. The vessels passed from the Pacific Ocean to the Sea of Japan through the Tsuruga Strait. 

The spokesman added that it is unusual for Russian ships to pass through the strait so close to Japanese territory. 

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Japan recently reiterated its claim over the disputed Kuril Islands, which have served as a point of contention between Japan and Russia. 

Russia seized the islands, formerly part of Hokkaido Prefecture, at the end of World War II. Japan has labored in the decades since to secure them again, recently stressing that it believes the islands remain "primordially Japanese" and labeled Russia’s ownership as "occupation against international order." 

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Japan has supported and enact strong sanctions against Russia in reaction to the invasion of Ukraine, most notably by joining the U.S. and European allies to remove Russian banks from the international SWIFT financial system in late February. 

Japan also froze assets belonging to President Vladimir Putin and other top Russian officials while sending $100 million in emergency humanitarian aid to Ukraine. 

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"The Russian invasion of Ukraine is a unilateral attempt to change the status quo and the act shakes the foundation of the international order. It’s an outright violation to international law and we strongly denounce the act," Prime Minister Fumio Kishida previously said. "Japan stands by the Ukrainian people who are fighting hard to defend their sovereignty and territory, their homeland and families."

Fox News’ Stephen Sorace contributed to this report.