The US State Department on Tuesday confirmed the death of a US citizen last week in the predominantly Druze region of Sweida, where hundreds of people have been reported killed in clashes.
State Department spokesperson Tammy Bruce said Hosam Saraya, adding that the US was providing consular assistance to the family. She declined to provide further details.
ABC News earlier reported that friends and family of Saraya, an American-Syrian citizen, confirmed that he and other relatives were among those seen in a video being gunned down.
US says 'there is no successor to Sharaa'
The US helped broker a ceasefire last week that brought an end to the fighting, which erupted between Bedouin tribal fighters and Druze factions on July 13.
Barrack said the stakes in Syria are dangerously high, with no succession plan or viable alternative to the country's new Islamist government.
"With this Syrian regime, there is no plan B. If this Syrian regime fails, somebody is trying to instigate it to fail," Barrack said. "For what purpose? There's no successor."
Asked if Syria could follow the dire scenarios of Libya and Afghanistan, he said: "Yes, or even worse."