Al-Nusra Front announced it has captured a major airbase in northwestern Syria after a two-year siege by the group.
The group also announced on Wednesday that it killed at least 100 government forces and captured 60 others, in addition to capturing large quantities of weapons.
In a newsflash on Wednesday, state television reported that the army garrison that had controlled the military airport in the province of Idlib had evacuated the post.
The airport has been under siege for almost two years by rebels who have captured most of the province.
Al Jazeera spoke to a source with knowledge of the Nusra Front's operations, who said the group had taken advantage of an ongoing sandstorm in the region to advance on the base.
"The regime couldn't shell around the area because their planes could not shell accurately and they couldn't see," the source said.
"Nusra took advantage of that situation and took control of large parts of the airport."
The source added that not all government forces had withdrawn and clashes were continuing.
The Nusra Front is part of the Fattah Army, a coalition of rebel groups which took control of Idlib city in March.
The UK-based Syrian Observatory for Human Rights said fighters, including members of the Nusra Front, captured "most of the" base on Wednesday, including the command centre.
Obai Shahbandar, a defence analyst, told Al Jazeera that the loss of the base was a blow to the Syrian government.
"The rebel strategy seems to be to encircle the Assad regime's forces and force them to withdraw ... this was certainly a military and political embarrassment for Bashar al-Assad," Shahbandar said.
Idlib borders Turkey, and Idlib city is the second provincial capital in the hands of anti-government forces.
Raqqa city is controlled by fighters from the Islamic State of Iraq and the Levant (ISIL) group, which is fighting both the Syrian government and Syrian rebels, including the Nusra Front.
At least 20 civilians were killed during US-led coalition air strikes in the western suburbs of Raqqa city, the UK-based observatory reported. ISIL fighters were supposed to have been in the areas bombed, but activists told the observatory ISIL forced civilians to stay in those areas.
In another development, ISIL launched a major offensive on Deir Ezzor military airport, the observatory reported, adding that tens of government forces have been killed in the attack.
'Russian military build-up continues in Syria'
Russia is pressing ahead with its military build-up in Syria and has sent armoured personnel carriers, tank landing ships and a small contingent of troops into the war-ravaged nation in recent days, a US official told AFP news agency on Wednesday.
Russia's activity is focused on Bassel al-Assad International Airport, south of Latakia on Syria's Mediterranean coast, and at the Russian naval facility in Tartus.
Foreign Ministry spokeswoman Maria Zhakarova accused the West of creating "strange hysteria" over Russian activities in Syria, saying that Moscow has been openly supplying weapons and sending military specialists to Syria for a long time.
Zhakarova told journalists on Wednesday that "Russia has never made a secret of its military-technical cooperation with Syria."
And she could "confirm and repeat once again that Russian military specialists are in Syria to help them master the weapons being supplied".
Source: Al Jazeera and agencies