Syrian rebels claimed they had downed a military warplane in the eastern province of Deir Ezzor on Monday, after state media said a jet had crashed after suffering technical problems.
"Yes, we can confirm that a MiG 21 has been downed," said the Free Syrian Army spokesman Kassem Saadeddine. "It was hit with 14.5 calibre anti-aircraft machinegun."
A Britain-based watchdog also backed the claim.
"Eyewitnesses in Muhasen in the countryside of Deir Ezzor confirmed that a MiG was downed today, after it was hit and went up in flames," said the Syrian Observatory for Human Rights.
Syria's state media said earlier that a warplane suffered a malfunction while on a routine training mission in the country's east.
"The pilot ejected and searches are now under way to find him," the official SANA news agency said.
The monitoring group said: "Eyewitnesses ... confirmed that it was not on a training patrol, and that it was flying low."
An amateur video posted on YouTube by activists and marked with a logo reading: "The revolutionary youth from the land of the Euphrates," shows a fighter jet apparently being hit, after the sound of heavy fire is heard.
A ball of fire erupts from the plane, followed by a trail of smoke. The jet continues to fly, as it appears the plane has been hit in the tail.
If confirmed, it would be the first time that the rebels have succeeded in downing a Syrian plane.
Syria's rebels have long asked the international community for anti-aircraft weapons and the slogan of its weekly protest last Friday was "arm us with anti-aircraft weapons."
In recent weeks, regime forces have used fighter jets to fire into several areas of the country, including Aleppo.