Anti-regime activists say jihadist fighters linked to al-Qaida fought with rebels to capture a Syrian military air defense base near Aleppo.
Videos posted online Friday, apparently shot inside the base, indicate that a group called Jabhat al-Nusra participated in the overnight battle for the base. The videos show fighters inspecting lines of large missiles.
Jabhat al-Nusra, or the "front for the protection of the people of the Levant," is a jihadist group with links to al-Qaida. The group has claimed responsibility for a number of bombings since the start of the Syrian uprising.
Two Aleppo-based activists and Rami Abdul-Rahman, the head of the Syrian Observatory for Human Rights, also said Jabhat al-Nusra fought in the battle.
According to One of the activists, Mohammed Abu Omar, four rebels were killed in the overnight battle.
Syria is dotted with air defense bases built for use in possible wars with Israel. Rebels have stormed such bases before but have not succeeded in deploying their complex weapons.
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.
Turkey scrambles fighter jets to Syrian border
Meanwhile, Turkey scrambled two fighter planes to the border with Syria on Friday after a Syrian military helicopter bombed the Syrian border town of Azmarin, a Reuters witness said.
There has been intense fighting between rebels and Syrian government forces this week in Azmarin and neighboring towns, an area strongly opposed to President Bashar Assad's rule.
Shells fired onto the town from surrounding hills sent thick plumes of smoke and dust rising into the air during fighting on Thursday and a voice amplified through loudspeakers, audible from the village of Hacipasa in Turkey's Hatay province, called on rebel fighters to surrender, warning of an air assault.
Fighting along Turkey's 900-km (560-mile) border with Syria has repeatedly spilled over into Turkish territory in the past week, with the Turkish army responding in kind to gunfire and mortar shells fired from Syria.
Turkish Chief of Staff General Necdet Ozel said on Wednesday his troops would respond "with greater force" if the shells continued to land on Turkish soil, and parliament last week authorized the deployment of troops beyond Turkey, heightening fears that Syria's civil war would drag in regional powers.
Syrian opposition activists estimate more than 32,000 people have been killed since March 2011, when the uprising against Assad's regime began.