(corrects name Jessop to Lister in line 69)
* Videos in April show BGM-71 TOW anti-tank rockets
* First notable U.S.-made arms spotted in Syria civil war
* Not seen as enough to change course of war for now
* Origin unconfirmed, activist says given by Washington
* Possible new Russian rockets, Iranian drone also appear
By Peter Apps
LONDON, April 15 (Reuters) - Online videos show Syrian rebels using what appear to be U.S. anti-tank rockets, weapons experts say, the first significant American-built armaments in the country's civil war.
They would signal a further internationalisation of the conflict, with new rockets suspected from Russia and drones from Iran also spotted in the forces of President Bashar al-Assad.
None of that equipment, however, is seen as enough to turn the tide of battle in a now broadly stalemated war, with Assad dominant in Syria's central cities and along the Mediterranean coast and the rebels in the interior north and east.
It was not possible to independently verify the authenticity of the videos or the supplier of the BGM-71 TOW anti-tank rockets shown in the videos. Some analysts suggested they might have been provided by another state such as Saudi Arabia, a U.S. ally, probably with Washington's acquiescence.
U.S. officials declined to discuss the rockets, which appeared in Syria around the same time Reuters reported that Washington had decided to proceed with plans to increase aid, including delivery of lower-level weaponry.
U.S. officials say privately there remain clear limits to American backing for the insurgency, given the widely dominant role played by Islamist militants. A proposal to supply MANPAD surface-to-air missiles was considered but rejected.
National Security Council spokeswoman Bernadette Meehan said the Obama administration was giving support she did not define.
"The United States is committed to building the capacity of the moderate opposition, including through the provision of assistance to vetted members of the moderate armed opposition," she said in response to a query over the rocket videos.
"As we have consistently said, we are not going to detail every single type of our assistance," she said.
While the number of U.S. rockets seen remains small, reports of their presence are steadily spreading, analysts say.
"With U.S.-made TOW anti-tank missiles now seen in the hands of three groups in the north and south of Syria, it is safe to say this is important," said Charles Lister, visiting fellow at the Brookings Institution Doha Centre and one of the first to identify the weapons.
The first three videos were posted on April 1 and 5, Lister said. While two have since been removed, one remains on YouTube.
He posted clearer still images on a blog for Huffington Post last week.
Several other arms experts and bloggers on the Syrian conflict have also reviewed the videos. They include Eliot Higgins, a Britain-based, self-taught arms and video specialist who blogs under the name "Brown Moses" and has emerged as one of the leading authorities on foreign firepower reaching Syria.
The rebel faction shown operating the U.S. missiles in the first videos, a relatively secular and moderate group called Harakat Hazm, declined comment. But an opposition activist based in southeastern Turkey who is a former member of Harakat Hazm said that they were provided by the Americans.
The Syrian activist, who identified himself as Samer Muhammad, said Harakat Hazm received 10 anti-tank missiles earlier this month near Aleppo and Idlib, two cities torn by heavy fighting near the northern border with Turkey.
He said that Harakat Hazm had launched five of those rockets to destroy four tanks and win a battle in the Idlib suburbs of Babulin and Salheiya, and this was the first time such U.S. arms had figured in Syria's fighting.
His information could not be confirmed independently.
SAUDI, QATARI SUPPLIES
More recent videos had shown the rockets in the hands of the Syrian Revolutionary Front and another group named Awliya wa Katalib al-Shaheed Ahmed al-Abdo, Lister said. Both are also seen as broadly moderate, in contrast with radical Islamists.
Western states have long been reluctant to make good on repeated talk of supplying weapons to Assad's foes, nervous of arms falling into the hands of jihadi militants or simply abetting more bloodshed in a conflict that has killed over 150,000 people and displaced millions over the past three years.
Lister said that if Washington were unwilling to supply TOW rockets itself, the most likely point of origin was Saudi Arabia which has thousands of anti-tank projectiles in its arsenal.
Under terms of the original sale, Riyadh would be obliged to tell Washington if it were transferring them to any third party.
"Considering the groups already seen with these missile systems and considering Saudis' already established reputation for providing weapons to moderate... groups, Saudi would seem the most likely candidate at this stage," Lister said.
The other major regional supporter of the rebels, Qatar, apparently do not hold such rockets in its regular military stores, analysts say, and may have bought Chinese weaponry from elsewhere, perhaps Sudan, for shipment to rebels last year.
Chinese-built HJ-8 anti-tank guided missiles remain a relatively common part of the rebel arsenal, according to Syria arms experts. HJ-8s first popped up largely in the hands of Islamist groups early last year, possibly coming from Qatar.
More recent shipments have been noticed in the hands of relatively secular insurgent factions and are believed by analysts to have been supplied by Saudi Arabia instead.
RUSSIAN ROCKETS, IRANIAN DRONES
Use of Chinese MANPAD anti-aircraft missiles by Islamist militants has dwindled in recent months, monitors say. Such missiles arrived last year, again believed to have come from Qatar, a development that particularly worried Western states.
"I suspect there's been two waves of Chinese weapons, the first from Qatar and the second from Saudi Arabia going to different groups," said "Brown Moses" blogger Higgins.
The United States and other Gulf Arab states have bemoaned Qatar's scattergun approach to arming rebel forces that has seen many weapons end up in the hands of fighters affiliated with al Qaeda linked and other radical Islamists. Qatari and Saudi officials will not discuss their Syria policy in detail.
Gulf states have also been alarmed by growing signs of support from Iran for Assad's military. The latest new piece of Iranian equipment to appear on the battlefield, an unmanned Shahed 129 drone photographed over Damascus, is said by Tehran to carry weapons as well as conduct surveillance.
Higgins said the other most significant development in Syrian conflict firepower this year had been the government's growing use of Russian-made BM-27 and BM-30 rocket launchers to deliver cluster munitions. While the former had long been known to be part of Assad't armouries, the latter was not. (Additional reporting by Mark Hosenball in Washington, Dasha Afanasieva in Istanbul and Gabriela Baczynska in Moscow; Editing by Mark Heinrich)