Cyber espionage and attacks from well-funded nations or terror groups are the biggest threats to the military's computer networks, a top US officer said.

Air Force General Kevin Chilton, who heads US Strategic Command, said he worries that foes will learn to disable or distort battlefield communications.

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Chilton told reporters that even as the Pentagon improves its network defences against hackers, he needs more people, training and resources to hone offensive cyber war capacity.

At the same time, however, he asserted that the US would consider using military force against an enemy who attacks and disrupts the nation's critical networks.

"Our job would be to present options. I don't think you take anything off the table when you provide options" to the defence secretary or president, in the wake of an attack, whether the weapon is a missile or a computer program, he said.

Chilton's comments shed the most light to date on the Pentagon's ongoing debate over how to beef up its abilities to wage and defend against cyber warfare.

And they came as the military is planning to set up a new cyber command at Fort Meade not far from Washington that would report to Strategic Command.

In a wide-ranging discussion of the military's cyber issues, Chilton said the Pentagon's unclassified networks are probed thousands of times a day, as hackers try to steal information on military programs or planning.

"I worry when I see that important information is taken from our networks," he said. To date, he said there have been no major attacks against the military's networks, only intrusions or efforts to steal data.

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Asked directly whether probes have been traced to China, al-Qaeda or other groups, Chilton declined to answer.

His biggest fear, however, is that enemies hack into military battlefield systems, and when a US commander sends out an order that says forces should go left, it is changed to say forces should go right. While most systems are classified and walled off, he said there are often ways to cross into those networks.

The other worry is more internal. When a soldier or sailor sits down at a computer, Chilton said "it's like he's stepping to the guard gate at his base," and can open the digital gate and let adversaries in.

Pentagon plans for a new command are a response to concerns that the offensive and defensive cyber operations are currently separate, and not as coordinated as they should be.

Chilton said he needs 2,000-4,000 more workers over the next five years to provide the expertise needed for both offensive and defensive cyber operations. He said it is not clear yet how much additional money he needs.

He said there is no timeline for the creation of a new cyber command, noting that the idea has not gotten final approval from Defence Secretary Robert Gates.

The military, Chilton said, must expand its ability to quickly remedy software problems, change computer configurations and to know who is on the network and what they are doing.

In some war fighting areas the US is dominant, but he said he cannot say the same for the digital battlefield just yet.

While enemies need only a computer to join the fight, the military has improved its defences to the point where it will take larger, more educated, well-funded organisations or nations to pose a real threat.