"Their level of effort against the Department of Defense is constant" while alleged Chinese attempts to steal corporate trade secrets has been growing, Rear Admiral Samuel Cox, the command's director of intelligence, told Reuters after remarks to a forum on the history of cyber threats.

The Office of the National Counterintelligence Executive, a U.S. intelligence arm, said in a landmark report a year ago that "Chinese actors are the world's most persistent perpetrators of economic espionage."

"It's continuing apace," Cox said. "In fact, I'd say it's still accelerating." He accused China of trying to "exfiltrate" Pentagon secrets, jargon for sneaking them out.

Asked whether any classified U.S. networks had been successfully penetrated - something not publicly known to have occurred - Cox replied: "I can't really get into that."

A spokesman for the Chinese embassy did not immediately respond to a request for comment. In the past Chinese officials have denied such accusations.

Cyber Command is responsible for defending Defense Department networks as well as mounting any U.S. offensive operations in cyberspace. It was created about two years ago as a unit of the U.S. Strategic Command, the outfit responsible for U.S. nuclear and space operations.

Cox said a proposal was moving forward to elevate the cyberwarfare unit to the status of a full unified combatant command. This would put it on the same footing as its parent Strategic Command and the Defense Department's eight other top-level military units.

The matter was headed to the secretary of defense and the president for a decision that possibly would come by the end of the year, he said.

Cox spoke after telling a conference hosted by the Atlantic Council think tank that the overall sophistication and danger of cyber threats is increasing at "an accelerating rate, not a linear rate."

"So the potential for these things to do destructive damage is very high," he said.

The United States is among the few countries reliably reported to have mounted a destructive keyboard-launched attack - against Iran's disputed nuclear centrifuges using malicious code known as Stuxnet that surfaced in 2010.

Army General Keith Alexander, who simultaneously heads Cyber Command and the National Security Agency, told a forum in July that unspecified foreign countries, hackers and criminal gangs contributed to a 17-fold jump in cyber attacks on U.S. infrastructure from 2009 to 2011.

Promoting Cyber Command in the military hierarchy would simplify its operations in cyberspace and boost its ability to work directly with U.S. government agencies, allies and coalition partners.

(Additional reporting by Andrea Shalal-Esa; Editing by Jim Loney)

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