"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 clarify which hardline factions were involved, but said the jihadist Hayat Tahrir al-Sham (HTS) is active in the area.

Russia also confirmed the pilot's death, but there was no immediate claim of responsibility from rebel factions in Syria.

"A Russian Su-25 aircraft crashed during a flight over the Idlib de-escalation zone. The pilot had enough time to announce he had ejected into the zone," the defence ministry said.

It said the area was "under the control of Al-Nusra Front fighters", the former Syrian branch of Al-Qaeda that makes up the bulk of the HTC.

"The pilot was killed in fighting against terrorists," it said, quoted by Russian agencies.

- 'Dozens' of strikes -

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.

Russia's intervention has swung the nearly seven-year conflict firmly in the favour of Moscow's long-time ally Assad.

Russia has been looking to still the fighting that has left some 340,000 people dead since 2011 with a major diplomatic push since the start of last year.

It agreed with fellow regime backer Iran and rebel supporter Turkey to set up four "de-escalation" zones around Syria, including in Idlib on the border with Turkey.