Videos posted online Friday apparently shot inside the base say Jabhat al-Nusra participated in the overnight battle for the base. The videos show fighters inspecting lines of large missiles.
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Two Aleppo-based activists and Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, also say Jabhat al-Nusra fought in the battle.
The base's capture is sure to fuel fears that Islamic extremists are playing a greater role after nearly 19 months of unrest in Syria. Western powers cite their presence as a reason not to arm the rebels.
Syrian opposition activists estimate more than 32,000 people have been killed since March 2011 when the uprising against President Bashar Assad's regime began.
Clashes were also taking place Friday at a military barracks close to Maarat al-Nuaman, a town on the main highway to the northwestern city of Aleppo which was seized by rebel forces earlier this week, the Syrian Observatory for Human Rights said.
Rapidly intensifying conflict (Photo: AFP)
The pro-opposition Observatory gave a death toll for Thursday of more than 260 people, including civilians and combatants on both sides in violence in the capital and the north, west and east.
It said 92 soldiers were killed on Thursday, which would be one of the highest daily casualty counts on the government side since the uprising against Assad broke out in March 2011.
The official SANA news agency also reported fighting nationwide and said dozens of rebels, which it called "mercenary terrorists", had been killed.
The reports could not be independently verified but they indicate a rapidly intensifying conflict, with the death tolls of the past several weeks far exceeding previous months.
Although international attention has focused on the Turkish border in the past week, Aleppo and the city of Homs – north of Damascus and near the border with Lebanon – are being fought over and clashes take place almost daily in the suburbs of the capital Damascus as well as in the countryside.
Turkey scrambled two fighter planes to the border with Syria on Friday after a Syrian military helicopter bombed the Syrian border town of Azmarin.
More than 260 people killed in clashes Thursday (Photo: AFP)
The Observatory said the air defense base seized by the rebels was located in al-Tana village by the Koris military airport on the road east from Aleppo to al-Raqqa.
Aleppo, Syria's largest city and commercial hub, has been contested since July. The rebel capture of Maarat al-Nuaman cut the highway between Aleppo and Homs, the main route for the government to resupply and reinforce the northern city.
SANA said government forces were mounting operations to clear Aleppo's Karm al-Jabal area of rebels on Friday.
More than 30,000 people have been killed in the conflict which started out as a popular uprising against four decades of Assad family rule then descended into civil war. The armed forces have relied heavily on air power and artillery to hold back the rebels.
Fighting has also spilled over the borders into Turkey, Lebanon, Jordan and the Golan Heights, raising concern that the fighting could spread across the region, now home to 340,000 Syrian refugees.
The Associated Press and Reuters contributed to this report