Hacker attacks on celebrity Twitter accounts have further raised concerns over security at the social networking and micro-blogging service.
In the latest attacks, hackers have targeted the accounts of 33 high-profile Twitter users, including US president-elect Barack Obama and pop star Britney Spears.
The message walls of the affected accounts were defaced with offensive or embarrassing messages, which have been removed by Twitter.
The attacks were carried out by hacking into administration tools that are normally accessible only by Twitter's technical support team.
These attacks are more serious than phishing because Twitter's systems potentially exposed every account to being taken over, said Graham Cluley, senior technology consultant at Sophos.
"This shocking start to the year for Twitter should send a stark warning to any online company holding details of its users that it needs to make certain it has proper security in place to prevent illegitimate access," he said.
Twitter said in a blog posting that it considered the breach to be very serious and that the support tools would be offline until they are made secure.
Referring to both attacks, the blog said Twitter's on-call team was able to attend to the matter quickly and prevent too many people from being affected.