Eurogroup head Jean-Claude Juncker warned Greeks not to turn their backs on the euro, saying in a newspaper interview that a win by anti-bailout radical leftists in a vote on Sunday would have "unforeseeable" consequences for the monetary union.

The radical leftist SYRIZA party is racing neck-and-neck with the conservative New Democracy party ahead of the election, which could decide if Greece stays in the eurozone and spread turmoil across global financial markets.

SYRIZA leader Alexis Tsipras has threatened to reject the punishing terms of the 130 billion euro ($164.12bn) bailout that has prevented Greece from bankruptcy.

"If the radical left wins - which cannot be ruled out - the consequences for the currency union are unforeseeable," Juncker, head of the group of euro zone finance ministers, told Austrian paper Kurier.