MOSCOW (Reuters) -Russia on Thursday accused Japan of treachery and complicity in theft after Tokyo last week signed an agreement to lend Ukraine over $3 billion for reconstruction, backed by the proceeds from frozen Russian assets.

Japan signed an agreement with Ukraine on April 18 for the loan of 471.9 billion yen ($3.3 billion). Under the terms of the arrangement, the loan will be repaid with proceeds from Russian assets worth tens of billions of dollars which have been frozen in the European Union because of the war in Ukraine.

The money is part of the G7's Extraordinary Revenue Acceleration for Ukraine (ERA) programme, which aims to provide Kyiv with $50 billion for reconstruction.

Russian Foreign Ministry spokeswoman Maria Zakharova told reporters at a briefing in Moscow on Thursday that the loan would damage already poor relations between Japan and Russia which are strained by Tokyo's support for Ukraine against what the Japanese Foreign Ministry calls 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 regarded by us as complicity in theft," said Zakharova.

"Tokyo has cynically expressed its hope that these actions will not have a negative impact on relations with Russia. We have to disappoint them - such actions will certainly and inevitably have a negative impact on relations with Russia.

"We regard such a step as extremely hostile, as treacherous. It is inadmissible under both legal and universal concepts."

Russia said earlier this month it saw no reason to discuss the possibility of signing a long-awaited peace treaty with Japan to formally end World War Two because of what it called Tokyo's unfriendly stance towards Moscow.

Soviet troops took control of four islands off Japan's Hokkaido - known in Russia as the Kurils and in Japan as the Northern Territories - at the end of the war and they have remained in Moscow's hands since. The territorial dispute has prevented progress on signing a treaty.

($1 = 142.3600 yen)

(Reporting by Dmitry Antonov; Writing by Andrew Osborn and Lucy Papachristou; Editing by Mark Trevelyan)