Sberbank and Alfabank are among several Russian financial services firms to have been bombarded with DDoS attacks this week, according to internet security firm Kaspersky.

The attacks began on Tuesday afternoon, targeting the web sites of at least five of Russia's top banks for hours at a time, says Kaspersky.

Botnets of tens of thousands of hijacked devices located in 30 countries were used to flood the sites with as many as 660,000 requests per second.

Sberbank says that the attacks did not disrupt its services and that it has experienced 68 similar hits so far this year, although the most recent ones are among the biggest it has encountered.