A Russian pilot was killed Saturday after his warplane went down over Syria's northwest province of Idlib and he clashed with Islamists 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 came down in a parachute.

He then clashed with rebels and was killed.

Abdel Rahman could not immediately confirm which hardline factions were involved, but said the jihadist Hayat Tahrir al-Sham (HTS) is 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.

Troops loyal to Syrian President Bashar al-Assad launched a fierce offensive on Idlib in late December, with backing by 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 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, leading to the worst crisis in ties between the two countries since the end of the Cold War.