The U.S. government issued a rare public warning that sophisticated hackers are targeting energy and industrial firms, the latest sign that cyberattacks 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 email 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.