Rebels in eastern Syria said they downed a government fighter jet today, but state television said the plane crashed due to technical difficulties.
The downing of a warplane would be a rare event for lightly armed rebels faced with the superior weaponry of President Bashar al-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 state news channel Syria TV said the plane crashed due to technical problems during a "regular training mission" and a search party was under way. State news agency SANA said the pilot had ejected from the plane before it crashed.
Anti-Assad activists uploaded videos on YouTube, said to be from the town of Mohassen in Syria's eastern province of Deir al-Zor and showing a warplane streak 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 as to whether the jet had been struck by rebel gunfire or an anti-aircraft missile.
It was also not possible to verify the location or date of the video.
An opposition source working with rebels in the area told Reuters the insurgents 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 armoury is made up mostly of assault rifles, explosives and rocket-propelled grenades, have said they are unable to compete with the army's air power.
Fighter jets have been seen lately firing rockets on rebel-held villages and the northern city of Aleppo.