The head of the US body responsible for combating cyber-attacks has resigned unexpectedly following a string of online assaults on the CIA and other government agencies.

Randy Vickers, the director of the US computer emergency readiness team (US-CERT), stepped down on Friday, according to a Department of Homeland Security email obtained by the Reuters news agency.

According to Reuters, the email did not disclose any reason for Vickers's resignation. The Department of Homeland Security declined to comment. Vickers, director of US-CERT since 2009, will be temporarily replaced by deputy director Lee Rock.

Vickers's resignation follows a number of online attacks on government websites including the Senate, the CIA and the FBI. William Lynn, the US deputy defence secretary, revealed earlier this month that a foreign intelligence service had stolen up to 24,000 computer files from a Pentagon supplier in March – one of the largest successful cyber-attacks on a US government agency.

The hacker collectives Anonymous and LulzSec targeted state websites after US intelligence agencies vowed to clamp down on the groups, resulting in a string of arrests worldwide.

Sixteen alleged members of the Anonymous collective were arrested last week, as part of the investigation. Two British teenagers, 19-year-old Ryan Cleary and an unnamed 16-year-old, have been arrested in the past month accused of involvement in the attacks.

The unnamed south London teenager – thought to go by the online alias "Tflow" – was released on bail on Friday following his arrest on Tuesday.

Washington-based US-CERT is responsible for the protection of US government computer networks, including those of the Pentagon and Senate.

• This article was amended on 27 July 2011. The original referred to the US computer emergency readiness team simply as CERT. This has been corrected.