* Military source says TOW missiles have impact on fighting

* Says more TOW missiles seen since late October

* Vetted rebels have received TOWs under CIA programme

By Tom Perry

BEIRUT, Nov 25 (Reuters) - A Syrian military source said rebels are making heavy use of U.S.-made anti-tank missiles paid for by Saudi Arabia and supplied via Turkey in recent weeks and the weapons are having an impact on the battlefield.

The so-called TOW missile is the most potent weapon in the arsenal of rebel groups battling President Bashar al-Assad, and has been seen in action more frequently since Russia intervened with air strikes on Syria on Sept 30.

A rebel group was shown using one of the guided missiles to destroy a grounded Russian helicopter in Syria on Tuesday.

Addressing the increased supplies of TOW missiles for the first time, the Syrian military source said they had an impact on the fighting, but played down their overall significance, saying the army was gaining ground.

"Through the course of the battles it became apparent that the terrorists have a bigger quantity of American anti-armour TOW weapons. They started using this weapon intensively," said the source. The Syrian government describes all the insurgents fighting it as terrorists.

"This weapon, TOW, of course affects the work of the armoured divisions. Certainly, it is a well-known American weapon whose impact is known: it is effective against armoured vehicles," the source said. "They use it heavily which indicates this weapon has become available to them."

The comments are a further indication of how increased military support for rebels from Assad's foreign enemies has helped the them confront a major, multi-pronged attack by the Syrian government and its allies Russia and Iran.

Senior sources close to Damascus told Reuters earlier this month that increased supplies of TOW missiles had slowed ground offensives by the Syrian army and foreign allies including Iran's Revolutionary Guards Corps and Hezbollah.

While the Syrian government has won back some ground, including south of Aleppo and in the northwestern province of Latakia, rebels have managed to advance in other areas including Hama province, where TOWs have been widely used.

SAUDI PAYS FOR MISSILES SENT VIA TURKEY

A representative of one rebel group supplied with TOW missiles said his fighters were not currently suffering from a shortage of the weapon, as they had earlier. He complained, however, that they still only had one launching tripod for the missiles. His group is fighting south of Aleppo.

TOW missiles have been supplied to rebels under a programme of military support for vetted Syrian groups that has in some cases included military training by the U.S. Central Intelligence Agency, including on how to use TOW missiles.

Reuters reported on Oct. 31 from Washington that the CIA, in collaboration with Saudi Arabia and Qatar, recently broadened the number of rebel groups to which it is clandestinely delivers weapons including TOW missiles.

It also reported at the time a significant new shipment of TOWs had been delivered in October to what the United States believes are relatively moderate Sunni rebels in the northwest.

Rebels grouped loosely in the "Free Syrian Army" have released numerous videos of their fighters firing TOW missiles in the weeks since the army and its allies launched their offensives in areas of western Syria that are crucial to Assad's survival. Russia's aerial bombardment began on Sept. 30.

The military source said the increased supply of the weapon had been noticed since late October, particularly in the area between Idlib and Hama provinces.

"Certainly when any party uses an effective weapon, it will have an impact, but not to a great extent. In rural eastern, southern and southwestern Aleppo, and in rural northern and northeastern Latakia, our army is advancing.

"But I said any advanced weapon will affect the plans, the method of fighting of any army, any military formation."

Russian President Vladimir Putin said last month the U.S. military was providing anti-tank and anti-armour weapons systems in Syria, and training gunners. He also said this was "a major mistake", and the weapons would "certainly fall into the hands of terrorist organisations". The remarks were published on the Kremlin's website.

The military source said: "The one paying is Saudi, and the one securing the supply is Turkey." (Additional reporting by Denis Dyomkin in Moscow; editing by Giles Elgood)

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