The US Federal Bureau of Investigation (FBI) has made a campaign against cyberthreats one of its "highest priorities," with China, Russia and Iran in the crosshairs, the bureau's chief said Thursday.

FBI Director Robert Mueller told the House intelligence committee that cyber-espionage constituted "one of the most significant and complex threats facing the nation."

When asked to name the worst offenders around the globe that pose a threat to the United States, Mueller said: "You have countries such as Russia and China, others, Iran perhaps, who have capabilities that we're alert to."

He singled out China for its capabilities in economic cyber-spying -- or targeting commercial data.

"Since 2006, we've had several dozen cases, investigations, prosecutions of individuals related to China who have undertaken economic espionage, ex-filtration of information and the like," the FBI director said.

Beijing has repeatedly denied any state involvement in cyber-attacks against government agencies and firms, including one against US Internet giant Google in early 2010 that sparked a row between the United States and China.

Mueller said cyberattacks had "impacted our military, other government agencies, the financial and telecommunications sectors, and other critical infrastructure."

"Addressing this cyberthreat will be among the FBI's highest priorities now and in the years to come," he told lawmakers.