Regime ally Moscow and rebel backer Ankara have held several rounds of talks aimed at averting an assault, but government troops have been massing near the rebel zone.

"Russian warplanes resumed bombing Idlib province after a 22-day pause," said Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights.

The air raids "came a day after rebel units in Idlib hit regime positions in neighbouring Latakia province, which killed three pro-regime fighters," Abdel Rahman told AFP.

Tuesday's bombardment hit several areas held by the jihadist-led Hayat Tahrir al-Sham alliance, among them the large town of Jisr al-Shughur, but also areas held by rival Turkish-backed rebels, including the town of Ariha.

Abdel Rahman could not immediately provide a death toll for the strikes.

Seized from government forces in 2015, Idlib and adjacent areas form the last major chunk of territory still in rebel hands.

The Syrian military has been deploying reinforcements to the zone for more than a month and Russian has stepped up its war rhetoric.

"We know that the Syrian armed forces are getting ready to solve this problem," President Vladimir Putin's spokesman Dmitry Peskov said on Tuesday, calling Idlib a "pocket of terrorism."

Moscow has been carrying out strikes in Syria since September 2015, using aircraft based at the Hmeimim base in Latakia province.

Russia accuses rebels in Idlib of attacking Hmeimim with weaponised drones and insists jihadist groups in the province must be eliminated.

Analysts say there is still a window of opportunity to avoid the humanitarian impact of a full-scale offensive.

The presidents of Turkey, Russia and fellow regime ally Iran are to meet in Tehran on Friday for three-way talks that are expected to focus on Idlib.

An estimated three million people -- half of them displaced from other parts of Syria -- live in the province and adjacent rebel-held areas.