Al-Qaeda's affiliate in Syria and allied rebel groups have taken control of two key army bases in the northern province of Idlib, activists say.
Members of al-Nusra Front, Jund al-Aqsa and units of the Free Syria Army (FSA), captured Wadi al-Deif base on Monday after launching an offensive on Sunday.
Ahrar al-Sham later joined their assault on the nearby Hamidiya base.
Forces loyal to President Bashar al-Assad had until now managed to repel a number of assaults on the facilities.
Situated outside the town of Maarat al-Numan and next to the country's main north-south motorway, they have been surrounded since 2012.
A Twitter account run by al-Nusra declared on Monday morning that "Wadi al-Deif was completely liberated" and said the group was now removing mines.
The Syrian Observatory for Human Rights, a UK-based activist group, said at least 31 government soldiers and 12 members of al-Nusra and Jund al-Aqsa had been killed in the fighting.
A local activist, Asad Kanjo, told the Associated Press that most of the troops at Wadi al-Deif had withdrawn before the base was overrun and headed to Hamidiya.
But within hours, the Syrian Observatory reported that Hamidiya had itself been captured by al-Nusra, Jund al-Aqsa and Ahrar al-Sham. The 13th Division and Fursan Al-Haqq - units of the FSA - also took part in the fighting to capture the bases.
At least 15 soldiers, including an officer, were taken prisoner, the SOHR added.
Rebels are now believed to control much of the countryside of Idlib province, while government forces dominate the provincial capital - also called Idlib - and the nearby Qarmid base.
The Syrian Observatory said al-Nusra and Jund al-Aqsa had used tanks and other heavy weapons which they captured from the Western-backed Syrian Revolutionary Front (SRF) last month.
The defeat of the SRF was seen as a blow to US efforts to build a moderate rebel force to counter jihadist groups like al-Nusra and the rival Islamic State (IS), which controls large parts of Syria and Iraq.
The latest fighting comes as European Union foreign ministers expressed their full support for a UN plan for a local ceasefire in the city of Aleppo.
On Sunday evening, the ministers met the UN special envoy to Syria, Staffan de Mistura, who believes the establishment of "freeze zones" will help de-escalate the conflict and allow for the delivery of aid.
Following talks in Brussels on Monday, the ministers said in a statement: "The EU will seek ways to provide practical support to his efforts, notably by contributing to the revival of local governance and administration, to the restoration of basic services and to the return to normalcy in areas of reduced violence, in particular in Aleppo, as conditions allow."
They also expressed concern about what they said was intensified military action against rebel forces by the government and warned that "cases of forced surrender imposed by the Assad regime through starvation sieges were labelled fallaciously as local cease-fires in the past".
Some 200,000 people have been killed since the uprising against President Assad erupted in 2011. Another 3.2 million people have fled Syria and 7.6 million have been displaced inside the country.