Jihadists shot down a Syrian warplane conducting strikes on the Islamic State group stronghold of Raqa on Tuesday, the Syrian Observatory for Human Rights monitoring group said.
"IS fighters fired on a military aircraft which crashed," the Britain-based group said.
"It is the first aircraft shot down since the regime launched air strikes against the jihadists in July following their declaration of a caliphate in late June," said the group, which relies on a wide network of doctors and activists for its reports.
Observatory director Rami Abdel Rahman told AFP that the plane was carrying out strikes on the IS stronghold of Raqa when it was hit.
It crashed into a house in the Euphrates Valley city, the sole provincial capital entirely out of Syrian government control, causing deaths and injuries on the ground, he added.
A photograph posted on a jihadist Twitter account purported to show the burnt-out wreckage of the plane.
"Allahu Akbar (God is greater), thanks to God we can confirm that a military aircraft has been shot down over Raqa," another account said, congratulating the "lions of the Islamic State."
The plane is far from the first Syrian government aircraft downed by opposition forces, but it comes after President Bashar al-Assad's regime stepped up its air campaign against IS in eastern Syria.
In recent weeks it has repeatedly targeted the group's Euphrates valley strongholds in Raqa and Deir Ezzor provinces and jihadist-held areas of the northeastern province of Hasakeh.
An air strike on an IS training camp in the Deir Ezzor town of Tibni killed 17 militants and a child on Saturday.