ATHENS--Greece announced Monday a one-day extension in its plan to buy back tens of billions of euros of debt--a move demanded by the country's official creditors in exchange for fresh aid--after intial offers last week fell short of target.

According to the country's debt management agency, the new deadline for offers--which had been previously set for last Friday afternoon--will now be extended until noon London time on Tuesday.

"We have decided to extend the invitation to offer designated securities for exchange to 11 December 2012," Stelios Papadopoulos, the head of the Public Debt Management Agency, said in a statement. "Holders that have not tendered so far can still take advantage of the liquidity opportunity offered by the invitation."

As of Friday's deadline, Greece had attracted between 26 billion euros and 28 billion euros ($33.6 billion and $36.2 billion) in offers, according to people familiar with the deal--just short of the roughly EUR30 billion the debt management agency had been targeting.

Since its financial crisis began in 2009, Greece and its official creditors from the euro zone and the International Monetary Fund have been struggling to bring down the country's crushing debt load. Earlier this year, Greece's private creditors--including the banks--were asked to take a steep write-down in the EUR200 billion of Greek debt they held. Despite that write-down, the country's debt burden is still expected to top 180% of gross domestic product this year.

The buyback seeks to retire about half the EUR62 billion in debt Athens owes private creditors. As part of the buyback plan, Greece is to use as much as EUR10 billion to buy back its outstanding bonds at around a third of their face value, thereby shaving off EUR20 billion of that debt.

Greece's four largest banks in terms of market capitalization--National Bank of Greece SA (ETE.AT), Eurobank Ergasias SA (EUROB.AT), Alpha Bank AE (ALPHA.AT) and Piraeus Bank SA (TPEIR.AT)--are all participating in the buyback, as is state-controlled lender Hellenic Postbank.

By re-opening the offer, the government hopes that Greece's banks will increase their pledges to ensure the program's success. The four big commercial banks, which hold almost a quarter of the nation's bonds in terms of value, still hold some EUR4 billion to EUR5 billion of that debt on top of the EUR10 billion they have already committed to sell--enough to cover the gap, the people said.

None of the banks has officially disclosed how many of their bonds they would sell back to the government. However, as of Friday, according to senior Greek banking officials, the country's four biggest banks had committed to sell just 67% of their total portfolio, hoping to hold on to the balance.

The buyback, which must be completed by Wednesday, is a precondition for unlocking the next tranche of aid for Greece from the euro zone and the International Monetary Fund. Thursday, euro-zone finance ministers are expected to give the green light to that next tranche of aid.

Write to Alkman Granitsas at alkman.granitsas@dowjones.com

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