Fighters from the al-Qaida-linked Nusra Front and other Islamist groups have seized two strategic army bases in northwestern Syria, giving them control over the largest outposts in Idlib province.
The Britain-based Syrian Observatory for Human Rights said militants from a group called Jund al-Aqsa helped the Nusra fighters take the Wadi Deif base in Idlib province and the nearby Hamidiyeh base.
Syrian government officials were not immediately available for comment.
Charles Lister of the Brookings Doha Center said the gains highlighted the rise of the jihadists in the province.
"The nature of the operations has served to underline the renewed prominence of more Islamically minded forces in Idlib, with Jabhat (Front) al-Nusra and Ahrar al-Sham having played the dominant role in practically capturing the facilities," Lister told the French news agency AFP.
The Observatory said at least 31 government soldiers and 12 opposition fighters were killed in the fighting that began Sunday.
Observatory director Rami Abdel Rahman said Al-Nusra had used tanks and heavy weapons captured last month from the Western-backed Syrian Revolutionary Front.
After Al-Nusra and its allies moved in, regime warplanes launched 17 airstrikes against the base, the Britain-based group said.
The latest round of fighting came as European Union foreign ministers met in Brussels on Monday to discuss ways to help implement a U.N. plan for a localized cease-fire in the northern Syrian city of Aleppo.
Opposition forces have carried out multiple sieges of the base since 2012.
The site is located about 1 kilometer from the main north-south highway in Syria, which links the key cities of Aleppo, Hama and Homs to the capital, Damascus.
Also Sunday, Syria's army seized an area north of Aleppo and killed 34 fighters from Islamist groups including al Qaeda's Nusra Front as fierce battles raged over the strategic territory, the Syrian Observatory and state media reported.
Pro-government forces captured an area east of al-Mallah farms outside Aleppo and is now aiming to secure parts in the west and cut off insurgent supply lines into the city, the activists said on Monday.
Syria's war started with a pro-democracy movement that grew into an armed uprising and has inflamed regional confrontations. The United Nations estimates about 200,000 people have died in the conflict.
Some material for this report came from Reuters, AFP and AP.