The Commission's analysis of Putin's edict found that it would create a new legal situation, and significantly change the current transaction procedure, a source told Bloomberg.

Additionally, the new process would introduce new costs to the buyer, which would be subject to Russia's preferences.

Most member states have said they won't be paying for any Russian energy supplies in rubles, however.

Meanwhile, Russia is becoming increasingly cut off from the global financial system. The bulk of its central bank's $600 billion in foreign currency reserves have been frozen.

Last week, Russia transferred rubles to make bond payments after the US Treasury blocked its attempt to pay in dollars.

Now, in the latest sign that Moscow is nearing a historic default, holders of Russian bonds have said they cannot accept rubles as payment.

Rating agency S&P Global last week put Russia in the "selective default" category, and in May, the world's biggest oil traders are set to cut out Russian oil purchases, according to a report.

(With inputs from agencies)