A suspected LulzSec hacker group ally arrested in Britain faces charges that he took part in cyber attacks on companies in the United States, court documents show.

Ryan Cleary was indicted by US prosecutors for conspiring to hack into computer systems to steal data and to overwhelm websites with simultaneous requests in what are known as distributed denial of service (DDoS) attacks.

An indictment naming the 20-year-old British man as "an associate of the hacking group LulzSec, an offshoot of the larger group Anonymous" was filed on Tuesday, according to the US attorney's office and FBI officials in California.

Cleary identified weaknesses in targeted computer networks for LulzSec colleagues, who were allowed to wage attacks using an army of virus-infected computers at his command, said US Attorney Andre Birott and Steven Martinez, assistant director-in-charge at the FBI field office in Los Angeles.

"Cleary assisted LulzSec in its hacking activities by identifying and exploiting security vulnerabilities on victim computers, conducting DDoS attacks, and providing access to servers and other computer resources for LulzSec members to use," Birott and Martinez said in a joint statement.

The indictment said Cleary worked with LulzSec members to attack computer systems at a news organization and several businesses during a three-month period in early 2011.

Cleary controlled a "botnet" of tens of thousands of computers infected with malicious software that let him seize command of the machines and turn them against targets by overwhelming websites with requests, according to prosecutors.

"He used his botnet to conduct DDoS attacks against various entities," Birott and Martinez said.

"The indictment further alleges that Cleary would rent out his botnet for certain time periods in exchange for money from individuals interested in conducting DDoS attacks targeting specific victims."

Cleary also used his resources to help LulzSec hackers communicate with one another or post stolen data online, according to prosecutors.

The criminal charges against Cleary carry a maximum sentence of 25 years in prison.

Cleary was among a handful of people arrested in Britain over links to the "hacktivist" group. He remained in British custody "while facing foreign prosecution in England" related to his hacking activities, US prosecutors said.