Russia has accused Japan of betrayal and complicity in theft, reports "Reuters".
This happened after Tokyo signed an agreement last week to provide Ukraine with a loan of more than $3 billion for reconstruction, secured by proceeds from frozen Russian assets.
On April 18, Japan signed an agreement with Ukraine for a loan of 471.9 billion yen ($3.3 billion). Under the terms of the agreement, the loan will be repaid with proceeds from Russian assets worth tens of billions of dollars that have been frozen in the European Union due to the war in Ukraine.
The funds are part of the G7's Extraordinary Acceleration of Revenues for Ukraine program, which aims to provide Kiev with $50 billion for reconstruction.
Russian Foreign Ministry spokeswoman Maria Zakharova said the loan would damage already poor relations between Japan and Russia, which have been strained by Tokyo's support for Ukraine against what the Japanese Foreign Ministry claims is "Russian aggression."
"We have long warned the Japanese side that participation in illegitimate experiments with the sovereign assets of the Russian Federation in any form will be viewed by us as complicity in theft," Zakharova said.
"Tokyo cynically expressed its hope that these actions will not have a negative impact on relations with Russia. We must disappoint them - such actions will certainly and inevitably have a negative impact on relations with Russia," she added.
"We assess such a step as extremely hostile, as treacherous. It is unacceptable both in legal and universal concepts," Zakharova was categorical.
Earlier this month, Russia said it saw no reason to discuss the possibility of signing a long-awaited peace treaty with Japan to formally end World War II due to Tokyo's "unfriendly position" towards Moscow.
At the end of the war, Soviet troops took control of four islands off Japan's Hokkaido - known in Russia as the Kurils and in Japan as the Northern Territories - and they have remained in Moscow's hands ever since. The territorial dispute has hindered progress on signing a treaty.