Five Russian banks have been under intermittent cyber-attack for two days, said the country’s banking regulator.
The state-owned Sberbank was one target of the prolonged attacks, it said.
Hackers sought to overwhelm the websites of the banks by deluging them with data in what is known as a Distributed Denial of Service (DDoS) attack.
Security firm Kaspersky said the attacks were among the largest it had seen aimed at Russian banks.
The data floods began on 8 November and have continued intermittently ever since, it added.
Most of the data deluges lasted about 60 minutes but the most persistent attack went on for almost 12 hours, the security firm said.
In a statement, Sberbank said it was able to neutralise the attack without affecting the ongoing operation of its website. It said it had suffered 68 similar attacks in 2016 but the ones in November ranked among the biggest it had faced.
The names of the other banks that were hit have not been released but all are believed to be among the 10 biggest in Russia.