Russia on Monday followed its ally China in suspending all Japanese seafood imports over Tokyo's release of wastewater from the crippled Fukushima nuclear plant.
Japan has been sharply critical of Russia's invasion of Ukraine, while Tokyo's relations with Beijing -- which has deepened ties with Moscow -- have worsened.
Rosselkhoznadzor, Russia's body responsible for regulating agriculture products, said it was "joining China's provisional restrictive measures on the import of fish and seafood products from Japan as of October 16, 2023" as a "precautionary measure".
It said the restrictions would remain in place "until the necessary exhaustive information to confirm the safety of seafood produce ... is forthcoming".
The Japanese government, which has called China's ban political and unscientific, urged Russia to reverse its decision, calling the move an "unjust" step "without any scientific basis."
The Japanese foreign ministry said it has explained the safety of the water release and complied with Russian requests this month for a meeting and further information about the subject.
"The decision by the Russian side is extremely regrettable, and we strongly request that it be withdrawn," the Japanese foreign ministry said in a statement.
In 2011, three reactors at the Fukushima-Daiichi nuclear facility in northeastern Japan went into meltdown following a massive earthquake and tsunami that killed around 18,000 people.
Twelve years on, Japan began in August to discharge treated contaminated water from the stricken plant into the Pacific Ocean.
China in response banned all Japanese seafood imports over the "selfish" and "irresponsible" release, accusing Japan of treating the sea like a "sewer".
- Big market -Japan insists the operation is safe, a view backed by the International Atomic Energy Agency (IAEA), the United Nations nuclear watchdog.
A team from the IAEA -- including a Chinese scientist -- is due to take water and fish samples from near the site this week.
Beijing says Tokyo has not proved the authenticity and accuracy of the nuclear wastewater data, nor that the ocean discharge of the water is harmless to the marine environment and human health.
Before the ban, China was Japan's biggest market for fish, accounting for more than $500 million worth of exports in 2022.
The Japanese government has promised additional aid for the fishing sector, while seeking to boost exports to Europe and elsewhere.
Russia is much less important for Japan, accounting for around 260 million yen ($1.7 million) worth of marine product exports in 2022.
A lot more seafood -- 155 billion yen -- went the other way from Russia to Japan.
- 540 Olympic pools -But the suspension is symbolically important as China and Russia grow closer in the wake of the Ukraine war.
Japan's close ally, the United States, has made a point of backing Tokyo, with the US ambassador to Japan publicly eating fish from the Fukushima area.
Japan banned imports of Russian lumber and vodka after the Ukraine invasion but marine products were exempted.
In all, Tokyo intends to discharge around 540 Olympic swimming pools' worth of water -- some 1.3 million cubic metres (345,000 gallons) -- from Fukushima in a gradual process lasting decades.
The water has been treated to remove radioactive substances -- with the exception of tritium -- and diluted with sea water.
Japan says tritium levels are within safe limits and below that released by nuclear power stations in their normal operation, including in China.
The water release is aimed at making space to begin removing the highly dangerous radioactive fuel and rubble from the wrecked reactors.