WASHINGTON, US - The US Defense Department said on Thursday it is reviewing the use of Facebook and other social media amid concern the sites pose a cyber security risk for sensitive military networks.

The announcement comes after the Pentagon had overcome initial reluctance and begun to embrace Facebook, Twitter and other social media, with even the country's top military officer, Admiral Mike Mullen, issuing his own "tweets."

"The concept of allowing access to social networking sites (SNS) on the Department of Defense .mil networks is currently under review at this time," Lieutenant Charlie Drey, a spokesman for US Strategic Command, said in a statement.

"It would be premature to comment on the outcome of the review."

Previously, the Pentagon worried the social networking sites could take up precious bandwidth or that careless entries could expose military secrets.

The concern driving the latest review was the free-wheeling nature of so-called Web 2.0 sites, which are more vulnerable to hackers and cyber assault.

The review was looking at "the requirements, vulnerabilities and risk mitigations" involved in social media and a new policy was being drafted on the use of so-called Web 2.0 sites, said Pentagon spokesman Lieutenant Colonel Eric Butterbaugh.

Wired magazine's blog Danger Room, citing unnamed military officials, reported Thursday that the Defense Department was preparing to issue a near-total ban on social media due to cyber security worries.

"There has been no decision from senior DoD (Department of Defense) leadership to block access across the board to all social networking sites," Butterbaugh said.

"However, as with any Internet-based capabilities, in addition to the merits and benefits, there are implementation challenges and operational risks that must be understood and mitigated," he said.

Strategic Command last week issued a "warning order" to the armed services asking for comments on a possible ban on social media on the Pentagon's unclassified network, NIPRNet, the Danger Room blog reported.

One option would allow some sections of the military - such as media and recruiting offices - to use computers connected only to the public Internet and not the Pentagon's own networks, the blog said.

Social networking sites are vulnerable to assault and are increasingly being targeted by cyber-criminals, experts say.

A vicious virus known as Koobface - "koob" being "book" in reverse - has affected thousands of Facebook and Twitter users since August 2008.

The virus hijacks the accounts of social networking site users and sends messages steering friends to hostile sites containing malware, a malicious software often designed to infiltrate a computer system for illicit purposes.