Thomson Reuters said the blogging platform of the Reuters News website was compromised on Friday and a false posting purporting to carry an interview with a Syrian rebel leader was illegally posted on a Reuters' journalist's blog.
"Reuters.com was a target of a hack on Friday," the company said in a statement. "Our blogging platform was compromised and fabricated blog posts were falsely attributed to several Reuters journalists."
One of the false posts purported to be an interview with Riad al-Asaad, the head of the Free Syrian Army.
"Reuters did not carry out such an interview and the posting has been deleted," the Reuters statement said.
In the purported interview, the FSA leader was alleged to have said his forces were pulling back from the northern province of Aleppo after clashes with the Syrian army.
The Free Syrian Army issued a statement denying that any such interview had taken place, and blamed President Bashar al-Assad's government for the false posting.
The report "was fabricated by the regime, as it seems the news agency was hacked", it said in a statement.
Thomson Reuters had no immediate information on who was behind the hacking.
Reuters journalists in Aleppo have reported Free Syrian Army fighters are still present in the city and outlying province.
The Reuters.com blogging platform was taken offline on Friday while the company works to address the problem.
(Writing by Claudia Parsons; Editing by David Storey)