One of Germany's biggest energy firms has said it is preparing to buy Russian gas using a payment system that critics say will undermine EU sanctions.

Uniper says it will pay in euros which will be converted into roubles, meeting a Kremlin demand for all transactions to be made in the Russian currency.

Other European energy firms are reportedly preparing to do the same amid concerns about supply cuts.

Uniper said it had no choice but said it was still abiding by EU sanctions.

"We consider a payment conversion compliant with sanctions law and the Russian decree to be possible," a spokesman told the BBC.

"For our company and for Germany as a whole, it is not possible to do without Russian gas in the short term; this would have dramatic consequences for our economy."