A Russian pilot was killed on Saturday after his warplane went down over Syria's north-west province of Idlib and he clashed with Islamist fighters on the ground, Russia's defence ministry and a monitor said.
"The pilot was killed as he fought Islamist rebels who had shot down his plane and were taking him captive," said the Syrian Observatory for Human Rights chief Rami Abdel Rahman.
According to the British-based monitor, the Sukhoi 25 was shot down over the town of Maasran in Idlib and the pilot parachuted down.
He then clashed with rebels and was killed.
Mr Abdel Rahman could not immediately confirm which hardline factions were involved, but said Hayat Tahrir Al Sham, led by a former Al Qaeda affiliate known as Al Nusra Front, was active in the area.
Russia's army also confirmed the pilot's death, but there was no immediate claim of responsibility from rebel factions in Syria.
"The pilot had enough time to announce he had ejected into the zone, under the control of Al Nusra Front fighters," the defence ministry said.
"The pilot was killed in fighting against terrorists."
Troops loyal to Syrian president Bashar Al Assad launched a fierce offensive on Idlib in late December, with backing from Russian warplanes.
"There have been dozens of Russian air strikes in the area over the past 24 hours. This plane was also carrying out raids there," said Mr Abdel Rahman.
Opposition factions have shot Syrian regime planes in the past, but downing Russian warplanes is much rarer.
In August 2016, a Russian military helicopter was shot down over Syria and all five people on board were killed.
Moscow began conducting air strikes in Syria in September 2015.
Two months later, Turkey shot down a Russian warplane that it said had entered its airspace near the Syrian border, leading to the worst crisis in ties between the two countries since the end of the Cold War.