STORY: Rosneft has asked for roubles and full prepayments in tenders for crude.

That’s according to sources close to the matter.

The move by Rosneft - Russia’s top oil exporter - puts it in line with President Vladimir Putin’s push for natural gas to be traded in roubles.

In late March, Putin ordered a switch to rouble payments for gas delivered by Russia's Gazprom.

Energy exports are the country's most powerful tool to hit back against Western sanctions.

Sources have told Reuters that for the May-June period, Rosneft has asked for 100% prepayment on 6.5 million tonnes of oil to be converted into roubles.

It has left some scope for payment in other currencies, though - including Chinese yuan and U.S. dollars.

But it also reserved the right to refuse deals where prepayment was not offered in full.

According to the sources, the rouble conversion would happen at the last stage of payment, and the initial price would still be nominated in U.S. dollars.

Two traders involved in the Russian oil markets said the terms were tough, but not impossible to meet.