The Defense Ministry said Thursday that it had spotted four large Russian amphibious warfare ships sailing through a strait in northeastern Japan as they traveled west, possibly toward Europe.

Pictures published by the Defense Ministry of the amphibious transports, typically used for landing expeditionary forces ashore, showed what appeared to be military trucks loaded onto the deck of one of the vessels.

"We don't know where they are heading, but their heading suggest it is possible" they could be headed for Ukraine, a Defense Ministry spokesman said.

A Self-Defense Forces maritime patrol first detected Tuesday the Russian vessels — which can carry dozens of tanks, other military vehicles and hundreds of troops — and monitored them as they moved west from the Pacific Ocean to the Sea of Japan through the narrow Tsuruga Strait separating Honshu from Hokkaido on Wednesday.

It is unusual for Russian ships to pass through the strait so close to Japanese territory, the spokesman said.

Armed with anti-tank weapons supplied by the United States and other countries, Ukrainian fighters have taken a heavy toll on Russian armor and fuel trucks. Moscow, which describes its attack as a "special operation," may be looking to reinforce its military with new equipment.

NATO allies, which have already supplied 20,000 anti-tank and other weapons to Ukraine, on Wednesday said they would keep helping the country resist the Russian attack.