On Monday, however, a Twitter account associated with Jabhat al-Nusra posted a photo of a TOW anti-tank guided missile system, a formidable weapon used against armored targets.

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In conjunction with the tweet, al-Nusra released a propaganda video showing fighters setting up what appears to be the TOW system, although there is no footage of them actually firing the weapon.

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It’s unclear if the weapons system was seized in the raid by al-Nusra last month. But Eliot Higgins, a prominent blogger who goes by the name Brown Moses online and who tracks open-source videos showing the fighting in Syria, claimed on his Web site that the model number on the missile tube, 71E-1B, is consistent with same type supplied to moderate rebels. He noted it remained possible that the fighters shown in the picture were not actually with al-Nusra, but concluded it was “likely” the TOW shown was captured from a vetted opposition group.

In the past, moderate factions have voiced skepticism of such claims and have insisted that all weapons provided by the United States and other allied forces are subject to a rigid accountability process to ensure that they do not fall into extremist hands.

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Oubai Shahbandar, a spokesman for the Syrian Opposition Council, said in an e-mail Monday there is “no compelling evidence that TOWs are being used for terrorist attacks targeting U.S. or allied personnel.” He also stressed the importance of not being influenced by what he called al-Nusra’s “psyops.”

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The BGM-71 TOW – short for a tube-launched, optically tracked, wire-guided missile system — has been in service in some capacity since the 1970s. While its long range and lethality makes it ideal for targeting tanks, its large size and heavy weight means it would be cumbersome to use for highly mobile ground forces such al-Nusra and other rebel groups. At the same time, the TOW system is relatively easy to use.

“All you have to do is put it together properly, which is not that hard, and put the missile in,” said Nicolas Ruggieri, a former TOW operator in Marine Corps.

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The fielding of a TOW system would not necessarily provide al-Nusra with a capability it didn’t already have. Similar weapons systems such as the Chinese HJ-8, French Milan and the Russian Spigot have all been seen in operation in Syria.

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“This weapon is not a game-changer for them,” said David Maxwell, associate director for Georgetown University’s security studies program.

“The big thing is the political aspect of American equipment falling into enemy hands and what that bodes for providing support to moderate rebels,” he added. “When you supply weapons to an indigenous force you have to be prepared for the fact it can be compromised and that has to be a fact of life.”

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Weapons experts say some arms have rapidly changed hands between various groups in Iraq and Syria. In September, a small-arms research organization cited evidence that “Osa” anti-tank rocket launchers originally intended for moderate Syrian rebels had been captured by fighters with the Islamic State.