Syrian troops and tanks had laid siege to Jisr al-Shughur after authorities vowed to retaliate against a deadly ambush that reportedly killed 120 security personnel in the town last week.

But refugees and rights groups said those killed were mutinous soldiers, shot for refusing to fire on civilians protesting against President Bashar al-Assad’s one-party rule.

Syria-based rights activist Mustafa Osso said advancing troops, using tanks, artillery and helicopters gunships, were fighting against hundreds of army defectors from the area. “This is the biggest and most dangerous wave of defections” since an uprising against Assad’s government began in mid-March, Osso said.

Several testimonies from defected soldiers have emerged in the last week, suggesting a split within the security forces.