None of the Russians targeted by European Union sanctions have declared their assets to German authorities as required to do under Germany's sanctions law, the German government said, prompting a call for the transparency regime to be tightened.

Some 4.28 billion euros in assets belonging to sanctioned oligarchs have been frozen in Germany since Russia's invasion of Ukraine, including 2.3 billion euros in deposits at German banks, according to the economy ministry.

Under Germany's sanctions law, targets of EU sanctions have a duty to declare their assets immediately, under penalty of a fine or up to a year in prison. But in a letter to Left party legislator Christian Goerke, the ministry said that none had yet done so.

"No declarations have been received by the Bundesbank or the Federal Office for Economy and Export Control," state secretary Udo Philipp wrote in the letter, which was seen by Reuters.

Goerke said the failure of the transparency regime showed that the rules needed to be tightened.

"The duty of transparency should be extended to people who do business with oligarchs, like notaries, brokers, used car dealers, art dealers and banks," Goerke said in a statement.

"It's not enough to put the names of oligarchs on sanctions lists," he added. "Putin's oligarchs are laughing like a drain while authorities are despairing of ever tracking down and freezing their assets."