Jihadist fire downed a Syrian air force plane over the Islamic State 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."

The Euphrates Valley city of Raqa is the sole provincial capital entirely out of government control.