Two news organisations which have employed the freelance journalist Austin Tice said the 47-second YouTube video did appear to show him, blindfolded, but that it lacked enough information to draw conclusions about his current condition or whereabouts.
The video was dated September 26 but only noticed on Monday by Tice's family and colleagues in the United States.
In the video, a blindfolded man who appears to be Tice is dragged out of a vehicle by several gun-toting men whose faces are not shown. He utters a few words in Arabic including "in the name of God, most gracious, most compassionate" and then "there is no God but Allah" before saying, in English, "Jesus, Jesus".
Tice had been working for The Washington Post and McClatchy Newspapers, which repeated calls for his release.
"We call on those who are holding Austin to release him promptly, unharmed," Marcus Brauchili, executive editor of The Post, said in a statement. "Austin is a journalist who was doing his job. He should be allowed to return to his family."
McClatchy vice president for news Anders Gyllenhaal said: "Austin Tice is a journalist, risking his life to tell the story of what's happening in Syria to the rest of the world. We ask in the strongest possible terms for his immediate release."
US officials said they were not able to verify the video but repeated that they believe Tice is being held by the regime of Syrian leader Bashar al-Assad.
McClatchy, a newspaper group, said Tice entered Syria in May without a visa, a common practice for journalists seeking to report on the war as Damascus issues few press visas. Many correspondents have crossed the border independently.
He then travelled throughout Syria with rebel forces, and reached the Damascus area in late July, setting up base in Daraya, which was pounded for days by Assad regime forces.
McClatchy said Tice last communicated with his colleagues on August 13, though he did not indicate how he planned to leave Syria.
