BEIRUT » Fighters from a shadowy militant group with suspected links to al-Qaida joined Syrian rebels in seizing a government missile defense base in northern Syria today, according to activists and amateur video.

It was unclear if the rebels were able to hold the base after the attack, and analysts questioned whether they would be able to make use of any of the missiles they may have spirited away.

Nevertheless, the assault underscored fears of advanced weaponry falling into the hands of extremists playing an increasingly large role in Syria’s civil war.

Videos purportedly shot inside the air defense base and posted online stated that the extremist group, Jabhat al-Nusra, participated in the overnight battle near the village of al-Taaneh, five kilometers (three miles) east of the country’s largest city, Aleppo. 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 today said Jabhat al-Nusra took the lead in the attack, killing three guards and taking others prisoner before seizing the base. The report showed a number of missiles and charred buildings, as well as fighters wearing black masks.

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 with other rebel groups. They disputed the notion that the extremist group had the lead role in the attack, however.

It was impossible to independently verify the videos and conflicting reports because of restrictions on reporting in Syria.

Despite Western opposition to President Bashar Assad’s regime, the U.S. and other countries have cited the presence of extremists among the rebels as a reason not to supply the Syrian insurgents with weapons. They have repeatedly cited concerns of heavy weaponry falling into wrong hands.

Rebel leaders argue that arms shortages mean they’ll take aid from whoever offers it, regardless of their ideology.

The capture of the base also plays into fears about extremists acquiring Syria’s chemical and biological weapons — particularly if the Assad regime collapses and loses control of them.

Neighboring Jordan’s King Abdullah II fears such weapons could go to al-Qaida or other militants, primarily the Iranian-allied Lebanese Hezbollah. The U.S. has sent about 150 troops to Jordan, largely Army special operations forces, to bolster the kingdom’s military capabilities in the event Syria’s civil war escalates.

Syria is believed to have one of the world’s largest chemical weapons programs, and the regime has said it might use the weapons against external threats, though not against Syrians.

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 Assad.

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-Qaida. While neither group has officially acknowledged a link to the other, analysts say al-Nusra’s tactics, rhetoric and use of al-Qaida forums point to an affiliation.

Activists on the ground say the group is known for fighting on the front lines in harsh battles and goes out of its way not to show up in activist videos.

“Most brigades want to be filmed in operations so they can get support, but al-Nusra doesn’t allow any filming,” Aleppo activist Mohammed Abu Omar said via Skype.

The base captured today 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 dozens of boxes of ammunition in the Daraa base.

The storming of these bases by the poorly armed rebels is an embarrassment to the Assad regime, but analysts say the missiles are unlikely to benefit the insurgents.

Pieter Wezeman of the Stockholm International Peace Research Institute identified the missiles in today’s video as S-75 surface-to-air missiles, which he said are old and hard to move and fire.

“They are outdated and difficult to operate, just not made for use by guerrilla forces not trained in using these things,” he said.

The regime, which has received much more advanced surface-to-air missiles from Russia in recent years, probably did not make defending the site a priority, he said.

It also remains unclear if the rebels held the base after storming it. Rebel forces are largely helpless against the regime’s attack jets and helicopters, which bomb rebel and civilians areas daily.

One Aleppo activist said the rebels had taken all the munitions they could from the 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.

In any case, it was not clear how much the rebels would be able to make use of the missiles.

“Anyone trying to use these will need to be extremely well trained both in fueling up the missiles and then tracking the target and using the fire control radar,” said Jim O’Halloran, an expert in air defense systems at IHS Jane’s.

Meanwhile, the fallout deepened from a Syrian passenger jet forced to land in neighboring Turkey, as Russia said the plane traveling from Moscow to Damascus was carrying radar parts that were being transported legally.

Russian Foreign Minister Sergey Lavrov insisted the plane was carrying a legitimate cargo of “electric equipment for radars,” but he added that it was of “dual purpose,” meaning it could have civilian and military applications.

“It’s not forbidden by any international conventions,” Lavrov said, adding that the Russian company that sent it to Syria will demand that Turkey return the cargo. He didn’t name the Russian company or the cargo’s recipient in Syria.

Russia has been Assad’s main supporter and ally, shielding him from international sanctions over his crackdown on the uprising.

Turkey’s prime minister has said the plane was carrying ammunition and military equipment for the Syrian Defense Ministry. Turkish fighter jets intercepted the Syrian Airbus A320 on Wednesday amid heightened tensions between Turkey and Syria, fueled by recent cross-border shelling from Syria that killed five Turkish civilians.

Tensions continued today as Turkey’s military scrambled two F-16 fighter jets after a Syrian attack helicopter was seen over a Syrian border town where rebels and regime troops have been clashing for days, Turkey’s Dogan news agency reported.

It said the jets were sent to the border to prevent a possible incursion into Turkey by the helicopter, which soon disappeared from view.

Turkey’s Foreign Ministry and military did not immediately confirm the report.

The Syrian Observatory for Human Rights said it recorded on Thursday its highest one-day death toll for government soldiers — 92 — since the start of the conflict.

It said most of the deaths took place in Idlib province, where some 20 soldiers were killed in a rebel attack on a government checkpoint.

Activists say more than 32,000 people have been killed as the conflict has evolved from a peaceful uprising to a brutal battle between rebels and government troops. Hundreds of thousands of Syrians have fled the fighting to neighboring countries.

Associated Press writers Elizabeth A. Kennedy in Beirut, Suzan Fraser in Ankara, Turkey, and Vladimir Isachenkov in Moscow contributed to this report.