TORONTO • The US government has issued a rare public warning that sophisticated hackers are targeting energy and industrial firms, the latest sign that cyber attacks present an increasing threat to the power industry and other public infrastructure.

The Department of Homeland Security and Federal Bureau of Investigation warned in a report distributed by e-mail late on Friday that the nuclear, energy, aviation, water and critical manufacturing industries have been targeted along with government entities in attacks dating back to at least May.

The agencies warned that hackers had succeeded in compromising some targeted networks, but did not identify specific victims or describe any cases of sabotage.

The attackers' objective is to compromise organisational networks with malicious e-mails and tainted websites to obtain credentials for accessing computer networks of their targets, the report said.

The US authorities have been monitoring the activity for months, which they initially detailed in a confidential June report. That document, which was privately distributed to firms at risk of attacks, described a narrower set of activity focusing on the nuclear, energy and critical manufacturing sectors.

The Department of Homeland Security declined to elaborate on the information in the report or say what prompted the government to go public with the information now. "The technical alert provides recommendations to prevent and mitigate malicious cyber activity targeting multiple sectors and reiterated our commitment to remain vigilant for new threats," it said.

The FBI declined to comment on the report, which security researchers said described an escalation in targeting of infrastructure in Europe and the US that had been noted in recent reports from private firms, including Symantec.

"This is very aggressive activity," said Mr Robert Lee, an expert in securing industrial networks.

Cyber security firm CrowdStrike said the technical indicators described in the report suggested the attacks were the work of a hacking group it calls Berserk Bear, which is affiliated with the Russian Federation and has targeted the energy, financial and transportation industries.

REUTERS