The Syrian Observatory for Human Rights said the air raids hit the Jabal al-Zawiya area on the edge of the jihadist-dominated northwestern province of Idlib.

In fighting on the ground, regime forces gained ground in the southern part of Idlib, the Britain-based monitor said.

They seized 10 towns and villages south of the M4 highway linking the coastal regime stronghold of Latakia to government-held second city Aleppo since Sunday, it said.

Nearly 50 fighters were killed on several fronts in the Idlib region, according to the Observatory, including 21 pro-regime fighters, as well as 27 jihadists and allied Turkey-backed rebels.

State news agency SANA, for its part, said "units of the Syrian army continued to progress in the south of Idlib" province.

Observatory chief Rami Abdel Rahman said the regime's aim was to wrest back control of stretches of the M4 still under the control of jihadists and allied rebels.

That would require operations against the towns of Ariha and Jisr al-Shughur, both along the M4.

Analysts expect a tough battle for Jisr al-Shughur, held by the jihadist Turkestan Islamic Party whose fighters mainly hail from China's Uighur Muslim minority.

They are allied to Hayat Tahrir al-Sham, a group led by Syria's former Al-Qaeda affiliate which dominates the Idlib region.

Loyalist forces have already taken back control of another key commercial artery running through northwest Syria -- the M5 that connects the capital with Aleppo.

They have also secured the region around the northern city, a major pre-war industrial hub.

Regime forces have since December clawed back chunks of the Idlib region, forcing close to a million people to flee their homes and shelters amid bitter cold.

The United Nations said Monday that the latest fighting was coming "dangerously close" to encampments of the displaced, risking an imminent "bloodbath".

Mark Cutts, a UN's humanitarian coordinator, also told reporters in Geneva that the world body was trying to double aid deliveries across a border crossing with Turkey from 50 to 100 trucks a day.

Syria's war has killed more than 380,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests.