BEIRUT - A video that shows a Syrian government fighter jet bursting into flames while under fire from rebels was posted on the Internet on Monday.
The downing of a warplane would be a rare event for lightly armed rebels faced with the superior weaponry of Syrian President Bashar Assad's forces.
In recent months the government has begun to use its air power to try to crush a 17-month-old uprising.
The video uploaded on YouTube was filmed by an opposition activist who said he was in the north-eastern town of Mohassen in Syria's Deir al-Zor province. It was not clear when it was filmed and it was not possible to verify the location.
The footage shows a warplane streaking through the skies amid heavy gunfire. The jet suddenly erupts into flames and begins to swirl, leaving a trail of smoke.
"God is greatest! A Mig fighter jet has been hit in the town of Mohassen," the activist shouted. There was no indication from the video that the jet had been hit by rebel gunfire or an anti-aircraft missile.
An opposition source working with rebels in the area told Reuters the rebels used anti-aircraft guns to down the jet.
"It was a Mig-21 brought down by a 14.5 anti-aircraft gun, the biggest in the rebel arsenal. The plane was flying too low and was within range. We have no information whether the pilot survived," the source said.
Rebels, whose armory is made up mostly of assault rifles, explosives and rocket-propelled grenades, say they are unable to compete with the army's air power.
In recent weeks, fighter jets have been seen firing rockets on rebel-held villages and the northern city of Aleppo.
Hours after rebels said that they had brought down the jet, the official news channel Syria TV - seemingly referring to the same incident - said a plane crashed due to technical problems during a "regular training mission." The state news agency SANA said the pilot had ejected from the plane and that a search party was underway.