Russia criticized the G7 countries' billion-euro loan to Ukraine, which is to be financed using the interest income from frozen Russian state assets.
"Using the funds effectively stolen from Russia for the warlike adventures of the Kiev regime is criminal and cynical and represents a further blow to international law," said Russian Foreign Office spokeswoman Maria Zakharova at a press briefing.
She threatened that the Russian reaction would be "painful" for Brussels.
The G7 states' plan would have fatal consequences, Zakharova said, adding such initiatives further undermine the global financial system and lead to destructive crises.
The plan will also have direct consequences for European companies in Russia, she said. "There is enough European property and capital, as you know - spokesmen of our government have spoken about this several times," she said, hinting at expropriations.
The Kremlin initially responded more cautiously. Russia will follow the G7 summit extremely closely, said Kremlin spokesman Dmitry Peskov, without commenting on the possible decisions rumoured in advance.
The G7 summit began on Thursday with an agreement to lend Ukraine $50 billion using money generated from frozen Russian state assets, in a major show of support for Kiev.