The Iraqi government appears to have blocked Facebook, Twitter and other social media, amid escalating combat.
Users in the country attempting to visit these sites are greeted by a message saying the Ministry of Communications has barred access.
The move comes after Islamist insurgents used Twitter to post a graphic image of a beheaded man, and to spread propaganda messages.
A Facebook spokesperson said the block was "a matter of concern".
One source in Iraq told the BBC that access to the internet was entirely restricted in some provinces.
Psiphon, a system which allows users to circumvent internet censorship, told the BBC it had seen a "huge influx" in the numbers of those using its service in Iraq.
On Sunday, more than 550,000 used Psiphon, up from about 8,000 on a normal day.
Cutting a 'lifeline'
Anti-censorship organisations have reacted angrily to the block, saying it harmed those using the sites for legitimate purposes.
Jillian York, director for international freedom of expression at the Electronic Frontier Foundation, which campaigns for civil liberties on the internet, told the BBC: "The Iraqi government will not achieve anything by blocking social media websites, rather, in doing so, they're cutting off a lifeline for activists and others to the outside world."
Commenting on the developments, a YouTube spokesperson said: "We're seeing reports that some users are not able to access YouTube in Iraq. There is no technical issue on our side and we're looking into the situation."
On Friday, Twitter's public policy unit tweeted: "Users in #Iraq are reporting issues accessing our service. We're investigating their reports and we hope service will be restored quickly."
A Facebook spokesperson said: "We are disturbed by reports of access issues in Iraq and are investigating. Limiting access to internet services - essential for communication and commerce for millions of people - is a matter of concern for the global community."
Although internet use was heavily controlled during Saddam Hussein's presidency, Iraqis have since enjoyed open access to the net.