BEIRUT, Lebanon — A commander in Syria’s elite Republican Guards, who was a member of the Damascus aristocracy and a close friend and contemporary of President Bashar al-Assad, was reported on Thursday to have fled the country and defected.

If confirmed, the defection of the commander, Brig. Gen. Manaf Tlass, would be the first from within the gilded circle around the president since the uprising against him began in March 2011, and the kind of embarrassing departure long anticipated to indicate that the government’s cohesion was cracking.

“Manaf is one of the regime’s main figures,” said Bashar al-Heraki, a member of the Syrian National Council, the umbrella political group in exile. Mr. Heraki, who sits on the council’s military coordination committee, said General Tlass would soon publicly declare his defection, but he declined to confirm reports that the general was in Turkey.

“It is a negative sign for this regime,” Mr. Heraki said. “It has started to lose control.”

Col. Riad al-Assad, the head of the Free Syrian Army in Turkey, a group of defected officers, said on Al Jazeera, the Arabic satellite channel, that General Tlass had fled to Turkey.