Britain-based war monitor says there have been dozens of Russian air strikes in the area over the past 24 hours.
A Russian pilot was killed on the ground in Syria on Saturday after parachuting into opposition-held territory when his plane was downed, Russian defence ministry said.
“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, under the control of al Nusra Front fighters,” the ministry said, quoted by Russian agencies.
The plane was shot down over the town of Khan al Subl near the city of Saraqeb, close to a major highway where the Syrian regime army and Iranian-backed militias are trying to advance, a source said.
TRT World's Mmalegabe Motsepe reports.
The Associated Press reports that the Russian pilot was shot and killed when he resisted capture by opening fire from his pistol on the militants who tried to seize him alive.
Russian officials are currently speaking with their Turkish counterparts to recover the pilot's body.
Russia's Defence Ministry said that the aircraft was downed by a MANPAD portable surface-to-air missile during a flight around the de-escalation zone in the province.
The US State Department said it had seen reports about the incident and denied allegations that the United States provided missiles to groups in Syria.
TASS news agency quoted the Russian Defence Ministry as saying Moscow retaliated with a strike that killed more than 30 militants in an area of Idlib province where the plane was downed.
The downed warplane had taken part in strikes that targeted civilian convoys fleeing along a major Syrian highway from villages that the army and foreign militias had overrun, according to news agencies.
According to Anadolu Agency, the plane was downed by the Free Syrian Army-affiliated Jaysh al Nasr. The spokesman of Jaysh al Nasr explained that they had shot down the plane the moment it carried out attacks in the eastern Idlib countryside.
At the same time, Reuters reported that the al Qaeda-affiliated Hayat Tahrir al Sham (HTS) claimed responsibility for shooting down the plane on social media, saying one of its fighters had scored a direct hit with a shoulder-launched anti-aircraft missile.
Russia in the Syrian war
Syrian regime 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 Abdel Rahman, head of the Syrian Observatory for Human Rights.
Opposition factions have shot Syrian regime planes in the past, but downing Russian warplanes is rare.
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.