Cybersecurity firm Kaspersky Lab said the distributed denial of service (DDoS) attacks represented the first major wave of such attacks on Russian banks this year and that at least five of the country’s largest banks had been targeted.

Russia has been on high alert for cyber threats after U.S. Vice President Joe Biden said last month that Washington would retaliate against alleged Russian hackers “at the time of our choosing” following what the United States said was a campaign of cyber attacks against Democratic Party organizations.

The source of this week’s attacks was unknown, however, and neither of the two Russian banks, nor Kaspersky, linked them to Biden’s comments.

Kaspersky said Russian banks were regularly subject to DDoS attacks, in which those conducting the attack flood the target with junk Internet traffic to disrupt its computer systems.

Sberbank said the DDoS attacks on its systems had started on Tuesday and continued on Thursday.

“The attacks were organized using botnets spread over tens of thousands of computers located in a number of different countries,” Sberbank said in a statement.

Alfa Bank said it suffered a short attack on Tuesday but that its business processes had not been affected.