Greece's central bank and police were on alert Monday to thwart any cyberattack against three Greek banks after a threat by hackers demanding bitcoin as ransom, officials said.

A police source said a group named Armada Collective had demanded 700 units of the virtual currency called bitcoins -- the equivalent of 15,000 euros ($16,000) to call off the incursion.

"It was a serious threat," the Bank of Greece source told AFP.

Kathimerini daily on Monday said Armada Collective had threatened last week to bring down the websites of three unnamed banks unless the ransom was paid today.

As a warning, the hackers on November 26 temporarily blocked the electronic banking services of the three banks, the daily said.

A counter-operation by cybercrime police and the Greek intelligence agency took nearly an hour to restore the system, the Bank of Greece source said.

Bank sources told Kathimerini that depositors' data were not in danger.

According to reports, Armada Collective has in the past made similar threats against web service providers in Switzerland and banks in Thailand.

Greece's four main banks -- National Bank, Piraeus Bank, Alpha Bank and Eurobank -- are in the process of recapitalisation after suffering a deposit flight earlier this year, forcing the government to impose capital controls in June.