A Department of Homeland Security (DHS) official responsible for defending US government networks against cyberattacks resigned on Monday.

Randy Vickers stepped down as director of the US Computer Emergency Readiness Team (US-CERT), the operational arm of the DHS's National Cyber Security Division.

Vickers' resignation was announced in an email to staff from Bobbie Stempfley, the DHS's acting assistant secretary for cybersecurity and communications.

A DHS official declined to provide an explanation for his departure saying the department does not discuss personnel matters.

But Information Week, which first reported Vickers' resignation, noted that it followed a string of cyberattacks on US government networks by hacker groups such as Anonymous and Lulz Security.

Lulz Security has claimed responsibility during the past few weeks for attacks on the websites of the Central Intelligence Agency, the US Senate, the Arizona Department of Public Safety and others.

US Deputy Defense Secretary William Lynn said earlier this month that a foreign intelligence service swiped 24,000 computer files from a US defense contractor in March in one of the largest ever cyberattacks on a Pentagon supplier.

The Washington-based US-CERT is responsible for the protection of US government computer networks and also cooperates on cybersecurity with the private sector and state and local authorities

Stempfley, in the email obtained by AFP, said Vickers' resignation was effective on Friday. She said he would be replaced by US-CERT deputy director Lee Rock until a new director is named.

"Lee has been the deputy director for US-CERT for over a year and we are confident that our organization will continue its strong performance under his leadership," Stempfley said. "We wish Randy success in his future endeavors."