The Islamic State group claimed its fighters shot down a Syrian regime warplane near the eastern city of Deir Ezzor and killed its pilot, in statements on jihadist websites.
The extremist group said the MiG fighter jet was downed over the south of Deir Ezzor city after it had carried out several raids on villages in the area.
The pilot died when the plane crashed, it said.
There was no immediate confirmation from the Syrian government.
But state-run SANA news agency reported that Syrian warplanes had carried out several sorties on Thursday to the east and south of Deir Ezzor city.
The Britain-based Syrian Observatory for Human Rights said that IS fighters shot down the regime plane as it flew over an area adjacent to a military airport.
IS controls large parts of Deir Ezzor city and most of oil-rich Deir Ezzor province, but has so far failed to overrun the airbase.
The Observatory -- which relies on a network of sources inside Syria for its information -- says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
It said the plane downed on Thursday was the fourth shot down by IS fighters since April.