The Russian parliament has said that any seizure of Russian assets by the European Union should be met with legal action against Belgium and the Euroclear stock exchange, Reuters reports.
Moscow could also seize the assets of investors from "hostile countries."
The Duma, the lower house of parliament, has passed a resolution containing the most explicit warning yet about how Russia will retaliate for an EU plan to use its frozen assets to finance a 140 billion euro loan to Ukraine.
Lawmakers said the proposal was "in practice illegal." seizure of property and therefore, if implemented, could be considered outright theft".
The resolution added: "Any attack on Russian assets must be met with an appropriate legal response, starting with claims for compensation — with a request for the seizure of assets as a precautionary measure — against Euroclear and Belgium, where the majority of illegally frozen state funds are held, in each jurisdiction".
"Assets of non-residents from hostile states could also be used as a source of compensation".
After Russia started a war in Ukraine in 2022, the West froze about $300 billion of its assets, mostly in Europe. Of this, €185 billion is held at Euroclear, a securities depository based in Belgium.
The EU plan would be structured to avoid a complete confiscation of assets - something that European authorities acknowledge could undermine investor confidence. It would involve lending to Ukraine using cash balances accumulated from frozen securities as they mature.
The scheme envisages that Kiev would repay the loan only after receiving war reparations from Russia in a peace deal - something Moscow has ruled out.
Ukraine has urged the EU to approve the plan quickly, saying it would use a significant portion of the funds to buy European weapons.
However, the plan has been delayed due to concerns from Belgium, which insists on strong guarantees that the scheme is legal and that other EU countries will share the risks involved.
At a summit last month, Belgian Prime Minister Bart De Wever called on all EU members to share the costs of any legal action taken by Russia and to contribute financially if the money ever has to be returned. He also said that frozen Russian assets held by other countries should also be part of the scheme.
The Kremlin has repeatedly warned that attempting to seize Russia's assets would undermine confidence in the central banking system, in the euro as a currency, and in the perception of the security of property and property rights in Europe.
Since the start of the war, Russia has seized assets worth tens of billions of dollars held by foreigners there, including the Russian businesses of Western companies such as French food company Danone and Danish brewer Carlsberg.
Moscow has also redirected an undisclosed amount of funds owned by foreigners in Russia to so-called type C accounts, access to which is blocked unless Moscow grants a waiver.