ATHENS--Greece will be blocked from the European Central Bank's debt purchasing program if a review by the country's international creditors isn't concluded, Greek Prime Minister Antonis Samaras said Thursday.

"[Thursday's] decision by the ECB says it clearly," Mr. Samaras said in a televised address. "Without winding up the review of our existing program by our international creditors, that must end in a month, by the end of February, we will be excluded."

His comments came shortly after the central bank's president, Mario Draghi, said the ECB will purchase eurozone countries" government bonds, a landmark decision aimed at combating stagnation and ultralow inflation.

Greece needs to stick to the terms of its bailout, footed by the European Commission, the European Central Bank and the International Monetary Fund, in order to be included in the ECB's program.

"While our policy guarantees Greece's full participation in this new era for Europe, the central position by the main opposition party keeps us with certainty out of this program," the Greek premier added.

Greeks are heading to the ballots Sunday, and according to polls the leftist Syriza party is poised to win. The party promises a tougher line with the creditors and wants Greece's eurozone partners to write off a substantial portion of the country's debt.

Greece's 240 billion euro ($274.2 billion) bailout program has been extended until the end of February.

The government that will emerge after Sunday's elections will have to conclude a review by the country's international creditors that has been pending since September and decide on the credit line that substitutes its existing program.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires