Greece's bailout during the financial crisis would be small change compared to what Ukraine needs to bolster its ailing economy, EU Energy Commissioner Guenther Oettinger said on Monday.
"If we want to help Ukraine -- a country with 45 million people -- then Greece was peanuts compared to that," Oettinger said during a visit to Bratislava, Slovakia, which neighbours Ukraine.
"It can't be done in one summer. It will take ten or fifteen years and will require a multi-billion-worth aid and participation of our taxpayers, voters, European Commission, USA, Canada, International Monetary Fund and others," Oettinger said.
"But if we're not willing and capable of helping them, the cost of a civil war and break-up of such a big country would be incomparably higher," he added.
Russia cut the flow of gas to Ukraine on Monday after last-ditch talks failed to end a dispute over debts that threatens to disrupt supplies to Europe for the third time in a decade.
Russia's state gas giant Gazprom said it had switched Ukraine to a pre-payment system, a move that effectively halts all shipments because Kiev and Moscow are in a dispute over unpaid debts.
The move tightens the screws on Ukraine as it struggles to mend its already weak economy while fighting a bloody separatist insurgency near its border with Russia.
In April, the IMF approved a $17 billion (12.5 billion euro) aid package for the former Soviet republic, but warned it may need to be recalibrated if Kiev loses control of the highly industrialised east.
That compares to the two bailout packages totalling 240 billion euros extended by the IMF and EU to crisis-struck eurozone member Greece since 2010.