BEIRUT, Lebanon — Syrian rebels said Monday that they had shot down a Syrian fighter jet for the first time, raising new questions about the opposition’s military capabilities, and whether Syria’s control of the skies might be threatened.

The Syrian authorities insisted that the jet had crashed because of a technical failure, but rebel groups and activists sought to win over skeptics by turning to YouTube. They posted one 33-second video showing a jet bursting into flames, and a second clip showing a man who identified himself as the ejected pilot, Farid Mohammed Suleiman. He told his captors in the video that he had been ordered to fire on an area in the eastern province of Deir al-Zour, and when an armed fighter beside him asked what he would like to say to the Syrian Army, he said, “I tell them to defect from this gang.”

The videos, shared widely online, seemed intended to provide a morale boost for rebel fighters, who have been complaining about the Syrian military’s undisputed air power for months. The videos set off another round of speculation about whether President Bashar al-Assad could maintain his military advantage in the 17-month-old conflict for much longer.

“Regardless of how they did it, if they can put down a jet fighter, then they can put down other planes as well,” said Sami Nader, an analyst and professor of international relations at St. Joseph University in Beirut. “The downing of the plane puts in place new rules of engagement and rules of dissuasion. The Free Syrian Army is showing us it can impose a no-fly zone. Assad’s trump card was the military, but he is now losing this last card.”