Twitter users were unable to "tweet" today after the social networking site became the victim of an attack.

A message posted on the Twitter blog said the short messaging site was the target of a "denial of service attack".

It said: "Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users."

The blog post added that Twitter was defending against the attack.

It was not clear exactly when the site went down, but it appeared to have become unavailable from mid-afternoon in the UK.

A message appeared on the site's status blog later in the afternoon, saying the site was back up - although it still appeared to be inaccessible.

The update said: "We are continuing to defend against and recover from this attack."

The status blog lets users know about planned outages and other issues on Twitter.