The US military is well equipped to defend the country against cyberattacks but is not yet ready to wage digital warfare, a senior defense official told lawmakers on Tuesday.

The military's cyber command, created in 2009, lacks the means to lead an offensive campaign in a fast-moving digital conflict, said Eric Rosenbach, the Pentagon's principal adviser on cyber security.

Asked by Senator Bill Nelson if the command lacks the computer network infrastructure to carry out a cyber offensive "effectively," Rosenbach said: "Yes, they currently do not have a robust capability."

But when it comes to defending US networks, "we are in good shape," he told a Senate Armed Services subcommittee on emerging threats.

His comments were unusual because officials previously have suggested the military was ready to lead an offensive digital campaign if necessary.

Rosenbach told senators there was no shortage of resources or funding for cyber command but there were technical and manpower problems that had to be tackled.

Pentagon chief Ashton Carter was following the issue closely and was ready to support more investment in cyber command as needed, he added.

Carter is reviewing a new cyber strategy for the military that is due to be released next week, he said.

Cyber command, in preparing options for offensive digital operations, was prepared to go after some civilian targets, Rosenbach said, "but in a very precise and confined way."

Any offensive action would adhere to the laws of war and seek to avoid causing civilian casualties, he said.

But in response to questions from lawmakers on what a possible offensive operation might entail, Rosenbach said he would prefer to discuss the subject in a closed door, classified hearing.

Experts and former officials have cited the computer worm Stuxnet that disrupted Iran's nuclear program in 2010 as an example of a suspected joint US-Israeli digital operation.

US cyber command is supposed to create a force of about 6,000 personnel for all the branches of the armed forces.

The Pentagon recently said it hoped to reach that by 2016 but Rosenbach said the military will not meet that objective until 2018. He blamed automatic budget cuts in recent years that had undermined training efforts.