Syria's al Qaeda affiliate on Monday seized the key army base of Wadi al-Deif in the country's northwestern province of Idlib from government forces, a monitoring group said.
"The Al-Nusra Front, backed by (Islamist group) Jund al-Aqsa, seized control of the Wadi al-Deif military base... after a fierce offensive that began yesterday morning," the Syrian Observatory for Human Rights said.
The jihadists' lightning victory in Wadi al-Deif leaves most of Idlib province, which borders Turkey, under Al-Nusra Front's control.
It is a show of force by the Al-Qaeda branch, which in November drove mainstream rebels seeking President Bashar al-Assad's ouster from Idlib province.
Mainstream opposition fighters had Wadi al-Deif under siege for around two years, but failed despite repeated attempts to take it over from government troops.
"The jihadists' advance has major symbolic importance, and it also shows the rebels that Al-Nusra Front really is in control of the area," said Observatory director Rami Abdel Rahman.
During its offensive on Wadi al-Deif, "the Al-Nusra Front used tanks and other heavy weapons that it captured last month from the (Western-backed) Syrian Revolutionary Front", Abdel Rahman told AFP.
Al-Nusra's defeat of the SRF was seen as a blow to US efforts to create and train a moderate rebel force as a counterweight to jihadists.
Idlib province was among the first areas to fall from government control, soon after the 2011 outbreak of a revolt against Assad's rule.