A hacking group has targeted three Greek banks three times over the last five days - demanding that each bank sends them 20,000 bitcoins (€6.74m).

If they do not comply with the group's directions they are threatening to take their websites offline - the affected banks have not been named.

The hackers call themselves the Armada Collective - they have been launching distributed denial of service (DDoS) attacks on the sites. These attacks overload websites with requests until their servers crash.

On Thursday of last week the hackers successfully disrupted electronic money transfers in all three banks and their threats are being taken seriously and investigated by cyber-experts in the Greek central bank and the Greek police's electronic crime unit.

The Financial Times reports that an official from the Greek police force confirmed that further attempts were made to disrupt the websites as the banks failed to engage with the group, and that security on the sites has been improved since Thursday and these subsequent attacks were unsuccessful.

"These attacks are extremely serious but we were able to boost security and add capacity with the help of local internet service providers," a senior Greek banker said.

Online banking enjoyed a surge in popularity in Greece during the country's prolonged debt renegotiations as people feared disruption in bank branches.

Hackers also referring to themselves as the 'The Armada Collective' have carried out similar attacks in Switzerland and Thailand in recent weeks. These incidents all follow the same blueprint as they threaten DDoS attacks and demand ransom payments through Bitcoin.

A security note from Akamai says that its "current attack activity doesn't strike us as monumental" - but adds that it is "a credible source of attacks going forward."