Calls have been mounting to find ways to support Kyiv's war effort by tapping the bank accounts, investments and other assets frozen after Russia invaded its neighbour in 2022.

Simply confiscating the funds is not seen as an option but the EU is working on ways to use the profits they generate, which alone could represent billions of euros a year.

"It is time to start a conversation about using the windfall profits of frozen Russian assets to jointly purchase military equipment for Ukraine," von der Leyen said in a speech to the European Parliament.

Kyiv says it desperately needs more military and financial assistance, with a fresh $60 billion US package stalled in Congress.

"With or without the support of our partners, we cannot let Russia win," von der Leyen told lawmakers.

"And the cost of insecurity -- the cost of a Russian victory -- is far greater than any saving we could make now."

The EU has frozen more than 200 billion euros ($220 billion) of Russian central bank assets, with about 90 percent held by an international deposit organisation in Belgium.

"Ultimately this is about Europe taking responsibility for its own security," said von der Leyen, whose speech addressed the broader question of European sovereignty and defence as Russia's war on Ukraine enters its third year.

"The threat of war may not be imminent, but it is not impossible," she said. "The risks of war should not be overblown, but they should be prepared for."

The commission chief separately congratulated Sweden on clearing the final hurdle to joining NATO, calling it a "historic step for this country and for our common security."