Facebook was down temporarily in parts of Europe on Wednesday, with users in some countries outside the region also reporting problems.
CERT.be, the federal cyberemergency team for Belgium, said Facebook was hit by a DDOS (distributed denial of service) attack.
The social network however said the site was unavailable in Europe because of technical difficulties, and had been restored, according to news reports.
Facebook did not respond to a request for comment on the outage.
The social network was unavailable for about an hour in Iceland, and came up again around 8:30 am GMT, said Birgitta Jónsdóttir, a member of the Icelandic Parliament.
The outage led some users on Twitter to speculate that Facebook was attacked by the Anonymous group in retaliation for the arrests by U.S. authorities of five alleged hackers.
"There is an ongoing DDoS (distributed denial of service) attack towards Facebook. Accessing your Facebook account can temporarily fail," CERT.be said in a message on Twitter.
The agency however said in an email that it can't confirm a DDoS attack on Facebook. "Our tweet this morning about the DDoS attack was based on earlier threats and the goal was only to inform our community about problems with Facebook," said spokesman Jeroen Gobin.
Users on Twitter in a number of locations outside Europe, including Tunisia and South Africa also reported problems with Facebook.
Jónsdóttir said in a Twitter message earlier that Facebook was down in most of Europe, Egypt, Turkey and Russia.
Tunisia Live, a Tunisian news website in English, reported that Facebook was unavailable in the country, but was later restored. Brandon Lorenzo, a Twitter user and model in South Africa, said that Facebook users in South Africa were affected for about six hours on Wednesday. There were issues with logging in and general use, he said.
Facebook had another outage in February, a day after filing for its initial public offering.