The Pentagon announced it has authorised the use of Twitter, Facebook and other so-called "Web 2.0" sites across the US military, saying the benefits of social media outweighed security concerns.

The decision, which comes at a time of growing concern over cyber-security, applies only to the military's non-classified network.

But it could mean big changes for large portions of the armed forces, including the Marines, which had selectively banned social media on work computers.

The Department of Defence also had bans in place since 2007 on accessing certain bandwidth-gobbling websites like YouTube on its network.

"The purpose of the policy is to recognise that we need to take advantage of these internet-based capabilities. These Web 2.0 tools need to be part of what we use," David Wennergren, a deputy assistant secretary of defence, said.

"And what we had were inconsistent approaches. Some websites were blocked and some commands were blocking things."

Social media are increasingly important for the US military. Admiral Mike Mullen, who as chairman of the Joint Chiefs of Staff is the top US military officer, has a Twitter feed with more than 16,000 followers.

US Southern Command offered operational updates via Twitter on relief activities in Haiti.

Defence Secretary Robert Gates, 66, has said that he wants to use social networking to help the Pentagon interact with US military members, many of whom are in their early 20s.

But opponents have cited the risks of information leaks, of opening gateways to hackers, along with a potential overload of precious bandwidth on the Defence Department's network.

The new policy says commanders will still need to defend against cyber-attacks and block access to online pornography, gambling and sites promoting "hate-crime related activities". It also allows commanders to temporarily limit internet access if the bandwidth is overwhelmed, a key caveat for US forces fighting the wars in Iraq or Afghanistan, or stationed in remote, rugged places around the globe.