BEIRUT: A shadowy extremist group believed to be linked to al Qaeda fought alongside rebels who seized a government missile defense base in Syria on Friday, activists said, heightening fears that extremists are taking advantage of the chaos to acquire advanced weapons.

Videos posted online Friday said to have been shot inside the base said the extremist group, Jabhat al-Nusra, participated in the overnight battle for the air defense base near the village of al-Taaneh, east of Aleppo in northern Syria. The videos show dozens of fighters inside the base near a radar tower, along with rows of large missiles, some on the backs of trucks.

A report by a correspondent with the Arabic satellite network Al-Jazeera who visited the base Friday said Jabhat al-Nusra had seized the base. The report showed a number of missiles and charred buildings, as fighters covered their faces with black cloths.

Two Aleppo-based activists and Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, also said Jabhat al-Nusra fought in the battle.

Little is known about Jabhat al-Nusra, or the Support Front, which began claiming attacks in Syria earlier this year in postings on jihadi forums often used by al Qaeda. While neither group has officially acknowledged the other, analysts say al-Nusra’s tactics, jihadist rhetoric and use of al Qaeda forums point to an affiliation.

Western powers - and many Syrians - worry that Islamist extremists are playing an increasing role in Syria’s civil war, which started in March 2011 as a mostly peaceful uprising against President Bashar Assad.

Activist say more than 32,000 people have been killed in the conflict.

Despite their opposition to the Assad regime, the US and others have cited the presence of extremists among the rebels as a reason not to supply them with weapons. Rebel leaders argue that the lack of military aid leaves a vacuum that extremists can exploit.

The base captured Friday is part of the large air defense infrastructure Syria has built across the country over the years, mostly for use in a possible war with archenemy Israel.

Last week, the rebels reported seizing another air defense base outside the capital, Damascus, as well as a base in the southern province of Daraa. Online videos show them torching vehicles and seizing boxes of ammunition in the Daraa base.

The storming of such bases by rebels from inside Syria embarrasses the Assad regime, though it is unclear if the rebels have the know-how to deploy these bases’ weapons.

Nor is it clear if the rebels are holding the bases after storming them.

Friday’s Al-Jazeera report said rebels were already preparing to withdraw, fearing airstrikes by regime jets – a threat rebels can do little about.

Rebel forces have been vulnerable to airstrikes by the Syrian military, though they have shot down a few attack helicopters and claim to have downed at least one warplane.

One Aleppo activist said the rebels had taken all the munitions they could from the newly captured base, and he hoped they could find a way to use the missiles against Assad’s air force.

“We have asked all countries to help us with anti-aircraft weapons and no one has, so hopefully these will help,” said the activist, Mohammed Saeed.