Russia's Gazprom (GAZP.MM) said on Tuesday there were "serious" risks to gas transit to Europe via Ukraine after Kiev failed to make a pre-payment.

Gazprom said in a statement that Ukraine had 219 million cubic meters (mcm) of gas left that would be used up in two days. With no new payment, Russia would be forced to stop supplies to Ukraine, Gazprom added.

"It creates serious risks for gas transit to Europe," it said. Europe gets around a third of its gas needs from Russia, with roughly half being pumped via Ukraine.

On Monday, Ukrainian state energy firm Naftogaz accused Gazprom of failing to supply gas that Kiev had paid for in advance, days after Moscow said it was sending some gas destined for Ukraine directly to regions held by rebels.

After cutting off Ukraine's gas for six months, Moscow resumed supplies in late-2014 when the two sides signed an interim agreement, under which Kiev would pay off some of its debts and pre-pay for supplies for the winter.

Naftogaz said the Russian firm had broken this deal by delivering only 47 mcm of a 114 mcm order that Kiev had paid for in advance last Thursday.

Last week Ukraine cut supplies of gas to eastern regions held by pro-Russian rebels, and Moscow began supplying gas to the separatist regions directly for the first time.

A Gazprom spokesman said the supplies to the rebel regions were being shipped under a contract with Naftogaz.