BRUSSELS, June 30 (Reuters) - Belgium's prime minister said on Friday the windfall profit from Russia's frozen assets in Europe could provide 3 billion euro ($3.27 billion) a year to rebuild Ukraine.

While there is political consensus among European leaders to use these assets to pay for the reconstruction, the legality of how to extract these profits is complex and still needs to be researched in depth.

"We're working on a method based on windfall profits ... If we find a stable legal platform we could use it for Ukraine," PM Alexander de Croo told journalists at the European Council summit.

"A windfall profit system will be developed and the current estimation is that the total returns could be 3 billion euro a year."

The bloc has said it has frozen more than 200 billion euros ($217.78 billion) of Russian central bank assets in reaction to Moscow's invasion of Ukraine. Another 30 billion euros of Russian oligarchs' private assets were also immobilised.

De Croo said the bloc could not touch the assets themselves without jeopardising the financial system since they are cross-maintained between central banks. A large portion of the frozen assets are in Belgium.

"The Commission is working on it. It needs to be seen what the right legal basis is to do but we will be coordinating between different countries," he said. ($1 = 0.9184 euros) (Reporting by Julia Payne; Editing by Alison Williams)