Nearly 200 combatants on both sides were killed in 24 hours when the Syrian branch of Al-Qaeda took two regime bases in Idlib province, a monitoring group said on Tuesday.
The Syrian Observatory for Human Rights, which is based in Britain but gleans its information from a wide network of activists and medics on the ground, said Al-Nusra Front attackers also captured more than 100 regime soldiers.
"There were at least 100 dead on the regime side and 80 among the attackers, killed in clashes, bombardments and by mines," Observatory chief Rami Abdel Rahman said.
Then on Tuesday, the air force kept up its raids against the Maaret al-Numan area near the two bases, killing at least 10 civilians, said the Observatory.
The jihadists and their allies took at least 120 soldiers prisoner, while another 100 fled south towards the town of Morek in the neighbouring province of Hama.
A video distributed by the Observatory shows five of the captured soldiers kneeling on the floor of a room, stripped down to their underwear and with their hands tied behind their backs.
The captors kick the men in the face and on their heads, slap them, and then one man whips them mercilessly with a hose as one of the soldiers cries out in pain.
Seizing the key Wadi al-Deif and Hamidiyeh military posts on Monday gave the jihadists control of most of the northwestern province, in a major blow to President Bashar al-Assad's regime.
The jihadists advanced on the bases in coordination with Islamist rebel groups Ahrar al-Sham and Jund al-Aqsa, the Observatory said, adding that a string of villages in the area also fell.
It was also another defeat for Western-backed rebels who were driven out of most of Idlib last month by Al-Nusra Front fighters.
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Idlib was among the first provinces to fall, soon after the March 2011 outbreak of the armed revolt against Assad's rule.
- Ceasefire negotiator killed -
Elsewhere in the war-torn country, regime warplanes hit the besieged district of Waer in the central city of Homs, killing at least 13 civilians, said the Observatory.
Among the casualties was a member of a delegation that had been in talks for a ceasefire with the government, as well as his wife.
Waer is the last rebel-held area of Homs, which as once known as the "capital of the revolution" against Assad.
Talks have been underway for months on a potential ceasefire between regime and rebel forces, but they have so far been fruitless.
The conflict began as a pro-democracy movement demanding his ouster, but later evolved into a brutal war after the regime unleashed a massive crackdown.
More than 200,000 people have been killed in nearly four years, and around half the population has been forced to flee.