Hacker group Anonymous strongly rejected suggestions on Tuesday that followers could cause a power outage by staging a cyberattack on an electricity grid.

The Anonymous statement came after The Wall Street Journal quoted the head of the US National Security Agency (NSA) as warning that hackers could have the ability within the next year or two to cause a limited power outage.

The newspaper said General Keith Alexander has not publicly expressed such a concern about the power grid but has provided his assessment in meetings at the White House and in other private sessions.

Anonymous, on various Twitter feeds used by the group, accused the NSA director of "alarmist rhetoric & fear-mongering."

"Why would Anons shut off a power grid? There are ppl on life support/other vital services that rely on it. Try again NSA. #FearMongering," Anonymous said in a message on @YourAnonNews.

In addition to rejecting the Journal report, Anonymous also organized a spam attack on the newspaper's Facebook pages on Tuesday, urging supporters to post comments denouncing the article.

The Journal said the "comment flashmob" was apparently started in Germany.

Anonymous last month briefly knocked the FBI and Justice Department websites offline in retaliation for the US shutdown of file-sharing site Megaupload and members of the group targeted the CIA website earlier this month.

In late 2010, Anonymous attacked the websites of Amazon, Visa, MasterCard, PayPal and others in retaliation for their decisions to stop working with Julian Assange's anti-secrecy site WikiLeaks.