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.”


What brought the jet down, however, was a matter of dispute.


Local activists said rebel fighters used a heavy antiaircraft machine gun that a local brigade had seized from a nearby military base. Qassem, an activist in the area, which is known as Mohassen and is about 15 miles from the city of Deir al-Zour, said the rebels had commandeered the weapon a month ago, and had used it once to bring down a helicopter.


Mr. Nader said the rebels could be lying. He said the rebels might not be admitting that they have antiaircraft missiles provided by international allies, because those allies did not want to be seen as fueling the conflict.


None of these accounts could be verified because of the limits on reporting in Syria, especially in Deir al-Zour, a city far from Damascus and Aleppo, where most of the recent fighting has been concentrated.


Even as one group of rebels in the eastern corner of the country described what they considered a major achievement, rebel commanders around Aleppo provided a different portrait of military struggle.


As shelling continued throughout the area, Abdel-Aziz Salameh, the head of the revolutionary council in Aleppo and rebel-held territory to the north, met briefly with fellow rebels in the basement of a nondescript building in Tal Rifaat, where commanders from the front came and went, providing updates and discussing plans. In an interview before leaving the building, he said that the rebels’ main problem was a shortage of ammunition, and that one of their primary tactics was trying to cut off supply routes to the large Syrian Army force in and near the city.


“Our fighters are doing well,” Mr. Salameh said. “But we are not able to give them enough ammunition. The whole problem with this revolution now is the shortage of ammunition.”


The battle for Aleppo, he said, would swing decisively in the favor of the anti-Assad fighters if they had sufficient ammunition and could sever the roads from Damascus, Raqqa and Aleppo’s international airport, used to resupply the government’s forces.


He also urged the West to take heed of the fighters’ success thus far, in battles they have fought largely alone. He said the West’s inaction had made the rebels lose respect for Europe and the United States.


“They must stand now, at this moment, beside the Syrian people,” he said. “The Syrian people will win this revolution, and the West will have lost the chance to stand beside us when we most needed them.”


Pressure is also building for Syria’s neighbors. On Monday in Jordan, dozens of young Syrian refugees tried to break down a fence and flee one of the country’s main refugee camps at the Syrian border, where conditions have been a source of tension.


Syrian activists and a Turkish official also said on Monday that Turkey had stopped letting Syrian refugees cross into Antakya, after refugee camps in the region reached their capacity of 60,000 people. Around 700 people were waiting together on the Syrian side of the border, a Turkish government official said.


“We are providing them the basics like food and blankets until construction of four more camps is completed in the next 10 days,” he said. Around 7,500 people were placed in student hostels near the border last week, and the camps under construction will be capable of housing 40,000, he said.


Turkey has signaled that it will create a protected corridor for humanitarian aid inside Syria if the number of Syrians who have headed into Turkey exceeds 100,000.


“We are rapidly reaching that barrier,” the official said.


In Damascus, the commander of the United Nations monitoring mission said two-thirds of its observers had left the country, with a week left before its mandate expires. The commander, Gen. Babacar Gaye, said only about 100 observers were left, concentrated in Damascus.


General Gaye strongly condemned Mr. Assad for using heavy weapons, which rebels say include jets firing on at least three cities. And he urged Mr. Assad’s government to trade its military mind-set for “a mind-set of dialogue.”