The New York Times called the attack on its website "a big deal" after a group claiming to be the Syrian Electronic Army caused disruption.
Marc Frons, chief information officer for The New York Times Company, said: “In terms of the sophistication of the attack, this is a big deal.
“It’s sort of like breaking into the local savings and loan versus breaking into Fort Knox.
"A domain registrar should have extremely tight security because they are holding the security to hundreds if not thousands of websites.”
An Australian company that registers domain names said it had been hit by hackers after the websites of the New York Times, Twitter and Huffington Post UK were affected.
Melbourne IT spokesman Tony Smith said a reseller's username and password were used to access several domain names on that reseller's account and then those domain names were changed.
"We are currently reviewing our logs to see if we can obtain information on the identity of the party that has used the reseller credentials, and we will share this information with the reseller and any relevant law enforcement bodies," Smith said in an email to the Associated Press.
"We will also review additional layers of security that we can add to our reseller accounts," he added.
Twitter and the Huffington Post UK confirmed they were hacked after the Syrian Electronic Army made claims it had taken over the sites.
Twitter said the hack led to availability issues for an hour and a half but that no user information was compromised.
The Huffington Post attack was limited to the blogging platform's UK web address, a spokesperson said.
The attacks came as the Obama administration considers taking action against the Syrian government.
The Syrian Electronic Army has claimed it is responsible for disrupting the New York Times' website and that it has taken over Twitter and the Huffington Post UK:
New York Times reporter Christine Haughney said the Times' website was down after its domain name registrar Melbourne IT was hacked.
Twitter spokeswoman Christina Thiry said the company was looking into the claims.
The New York Times' website was inaccessible this evening, likely due to a "malicious external attack," spokeswoman Eileen Murphy said:
Though users can still access the newspaper's site by typing its internet protocol address, 126.96.36.199, into a web browser, typing nytimes.com produced error messages.
Two weeks ago, the Times' website suffered an outage that the company blamed on a server problem.
Twitter has issued a message to reassure its users after the social networking site detected "unusual access patterns" that led to "unauthorized access attempts to Twitter user data".
In a statement on Twitter's blog it said that its investigation had detected that attackers may have had access to usernames, email addresses and passwords for around 250,000 users.