Russian President Vladimir Putin has said that, from tomorrow, countries buying gas from Russia must pay in roubles, or their contracts will not be honoured.
In a televised announcement on Thursday, Mr Putin said he had signed a decree to that effect, adding that the switch to roubles was designed to "strengthen our sovereignty" and protect Russian jobs.
But the move was rejected by European capitals, with Berlin saying it amounts to "blackmail".
Mr Putin said: "In order to purchase Russian natural gas, they must open rouble accounts in Russian banks. It is from these accounts that payments will be made for gas delivered starting from tomorrow.
"If such payments are not made, we will consider this a default on the part of buyers, with all the ensuing consequences."
He added: "Nobody sells us anything for free, and we are not going to do charity either - that is, existing contracts will be stopped."
The Greek gas grid operator will review its plans on additional cargoes of liquefied natural gas that Greece might need if Russia halts gas supplies to the country, the energy ministry said on Wednesday.
The demand by Russia over payments in roubles for its gas, which meets a third of Europe's annual energy needs, has raised fears that it could turn off gas supplies.
