Micro-blogging sensation social media site Twitter disappeared for several hours this afternoon, with reports confirming loss of the service around the world.

Twitter's Status Blog first stated: "Site is down - We are determining the cause and will provide an update shortly.”)

Later the blog was updated to claim that Twitter is "defending against a denial-of-service attack, and will update status again shortly"

By 16:00 BST Twitter reported that "the site is back up, but we are continuing to defend against and recover from this attack."

At the time of writing Twitter was available but somewhat intermitently.

It's not clear when the outage started, but reports on the outage surfaced around 14:00 BST, with many users finding themselves unable to access the micro-blogging platfo

Twitter DOS Attack

A denial-of-service attack (DOS attack) involves commanding other computers to bombard a Web site with requests for data, causing the site to stop working. Hackers use botnets - or groups of computers they've infected with malicious software - to launch an attack.

Graham Cluley's Sophos blog describes it as "a bit like 15 fat men trying to get through a revolving door at the same time - nothing can move."

It's difficult to trace who controls botnets, as the networks involve compromised computers located around the world.

"On this otherwise happy Thursday morning, Twitter is the target of a denial of service attack," said company co-founder Biz Stone on the company's blog.

"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."

Follow PC Advisor on Twitter (when it's back...)

DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways - and now, apparently, Twitter.

Last month major public and private Web sites in South Korea were taken off the Internet by a series of distributed denial of service attacks.

More to follow.

London is particularly hard hit by the attack, being named last night as "the capital city of Twitter" by Twitter CEO Evan Williams.

Additional reporting by Siobhan Chapman, Computerworld UK, and Jeremy Kirk.