BEIRUT — Five civilians died on Monday in Russian airstrikes backing Syrian regime forces as they chip away at the country's last major rebel bastion, a war monitor said.

The Syrian Observatory for Human Rights said the deadly raids hit the area of Jabal Al-Zawiya in the south of the jihadist-dominated northwestern stronghold of Idlib.

The Britain-based monitor says it determines who carried out an airstrike according to flight patterns, as well as ammunition and aircraft involved.

It said regime forces had rapidly gained ground in the southern part of Idlib in the past 24 hours.

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

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

Observatory chief Rami Abdel Rahman said the regime's ultimate aim was to wrest back parts of the M4 still under control of the 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.

In recent weeks, pro-Damascus forces have taken back control of another key commercial artery running through northwestern 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.

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. — AFP