Nine Bashar Assad regime fighters were killed in Turkish shelling in northwest Syria, where Ankara-backed opposition fighters are fighting off advancing regime forces, a monitor said.
Syrian regime forces have since December taken parts of the last major opposition bastion of Idlib through violence that has displaced almost a million people.
Fighting raged on Monday, killing almost 100 fighters on both sides around the last opposition bastion, the Syrian Observatory for Human Rights monitor said.
Those included 41 pro-regime fighters, as well as 53 opposition fighters.
Overall on Monday, the regime advanced rapidly in the south of the bastion but lost the town of Nairab along the M4 highway to the Turkish-backed Syrian National Army (SNA) in the southeast.
Turkish shelling in that area killed four regime fighters near Nairab and another five near the town of Saraqib to its east, the Britain-based Observatory said.
Opposition fighters had already broken back into Nairab last week after the regime seized it at the start of the month, but then lost it again several hours later.
Saraqib, which lies at the intersection of the M4 and another important highway the M5, has been under regime control since Feb. 8.
Earlier Monday, Russian airstrikes killed five civilians in the Jabal al-Zawiya area in the south of the bastion, the Observatory said.
In fighting on the ground, regime forces seized 10 towns and villages south of the M4, which links the coastal regime stronghold of Latakia to the regime-held second city of Aleppo, it said.
State-run Syrian Arab News Agency (SANA), for its part, said "units of the Syrian army continued to progress in the south of Idlib" province.
U.N. warns of 'bloodbath'
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 the opposition
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 Turkistan Islamic Party whose fighters mainly hail from China's Uighur Muslim minority.
They are allied with the Hayat Tahrir al-Sham, a group led by Syria's former al-Qaida affiliate that dominates the Idlib region.
Loyalist forces have already taken back control of the M5, which connects the capital with Aleppo.
They have also secured the region around the northern city, a major pre-war industrial hub.
Fighting in northwest Syria since December has forced some 900,000 people to flee their homes and shelters amid the 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 U.N. 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.