By Alexei Oreskovic

San Francisco - Twitter and Facebook, two of the Web's hottest hangouts, suffered service problems on Thursday, raising speculation that they had come under a pre-planned co-ordinated attack.

Twitter, the popular micro-blogging service, was knocked down by a malicious attack that prevented people from accessing its website for several hours on Thursday.

Facebook members saw delays logging in and posting to their online profiles. The social networking site is working with Twitter and Google to determine whether there was foul play, a person familiar with the company said.

The Twitter outage follows a wave of similar cyber attacks in July that disrupted access to several high-profile US and South Korean websites, including the White House site. South Korea's spy agency said at the time that North Korea might have been behind the attacks.

Twitter co-founder Biz Stone said on Twitter's blog that the site was the victim of a denial-of-service attack, a technique in which "crackers" overwhelm a website's servers with communications requests.

"We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," Stone wrote.

A separate Twitter status Web page said: "As we recover, users will experience some long load times ... We're working to get back to 100 percent as quickly as we can."

The time stamp was listed as "three hours ago," as of 2:05 p.m. EDT (1805 GMT). - Reuters