Media companies including the New York Times, Twitter and the Huffington Post lost control of some of their websites after hackers supporting the Syrian government breached the Australian internet company that manages many major site addresses.
The Syrian Electronic Army, a hacker group that has previously attacked media organisations that it considers hostile to the government of Syrian president Bashar al-Assad, claimed credit for the Twitter and Huffington Post hacks in a series of Twitter messages late on Tuesday.
Security experts said electronic records showed that NYTimes.com, the only site with an hours-long outage, redirected visitors to a server controlled by the Syrian group before it went dark.
New York Times spokeswoman Eileen Murphy tweeted the "issue is most likely the result of a malicious external attack", based on an initial assessment.
The Huffington Post attack was limited to the blogging platform's UK web address. Twitter said the hack led to availability issues for an hour and a half but that no user information was compromised.
The attacks came as the Obama administration considers taking action against the Syrian government, which has been locked for more than two years in an increasingly bloody struggle against rebels.
In August, hackers promoting the Syrian Electronic Army simultaneously targeted websites belonging to CNN, Time and the Washington Post by breaching a third party service used by those sites.
The Syrian Electronic Army, or SEA, managed to gain control of the sites by penetrating MelbourneIT, an Australian internet service provider that sells and manages domain names including Twitter.com and NYTimes.
Officials at the New York Times, which identified MelbourneIT as its domain name registrar and the primary hacking victim, warned its employees to stop sending sensitive e-mails from their corporate accounts.
MebourneIT spokesman Tony Smith said that login credentials from one of its resellers had been used improperly.
Once MelbourneIT was notified, he said, the company restored the correct domain name settings, changed the password on the compromised account, and locked the records to prevent further alterations.