(Reuters) - As evidence mounts of Islamic militant forces among the Syrian opposition, senior U.S. and European officials are increasingly alarmed by the prospect of sophisticated weapons falling into the hands of rebel groups that may be dangerous to Western interests, including al Qaeda.

In an interview with Reuters, U.S. Defense Secretary Leon Panetta articulated U.S. worries that shoulder-fired anti-aircraft missiles, also known as MANPADS, could find their way onto the Syrian battlefield.

Intelligence experts believe that hundreds, if not thousands, of such weapons were looted from arsenals accumulated by late Libyan leader Muammar Gaddafi, and are floating on the Middle East black market.

"I think it´s fair to say that we have a concern about the MANPADS coming out of Libya," Panetta said in the Thursday interview. "We´ve had an ongoing view that it was important to try to determine where these MANPADS were going, not only the concern that some of them might wind up in Syria but elsewhere as well," he said.

Panetta added that he had seen no direct intelligence yet that such missiles had made their way to Syria. He did not specifically cite the rebels as potential recipients.

But other U.S. and allied officials voiced that concern, while saying they had no evidence that Syrian rebels had yet acquired MANPADS.

SIGNS OF AL QAEDA JOINING REBELS

The urgency of Western concerns stems as much from the recipients of the weapons as the weapons themselves. High-level sources at multiple national intelligence services report increasing evidence that Islamic militants, including al Qaeda and its affiliates and other hard-line Sunni groups, had joined forces with opponents of the government of President Bashar al-Assad.

Bruce Riedel, a former CIA officer who has advised President Barack Obama on counter-terrorism policy, said that al Qaeda and other militants were "deeply engaged" with anti-Assad forces. He cited public pronouncements by senior al Qaeda figures, including the group´s leader, Ayman al Zawahiri, that urged Sunni rebels in Syria to kill members of Assad´s Alawite Muslim minority.

A western government source said that Al-Nusrah, a "spinoff" from al Qaeda´s Iraq-based affiliate, was responsible for at least some atrocities that have occurred in Syria. The source said the group publicly confirmed its role in killings.

Worries that sophisticated weapons could make their way to the wrong kind of Syrian rebels are one reason Washington remains wary of deeper U.S. involvement in the fighting.

"It stands to reason that if any Middle Eastern nation is even considering giving arms to the Syrian opposition, it would take a measured approach and think twice about providing arms that could have unintended consequences," a U.S. official said.

Nonetheless, U.S. and allied officials say their Saudi and Qatari counterparts have discussed how MANPADS could be used by Assad opponents to bring down Russian-made helicopters the Syrian army is using to redeploy its troops rapidly between trouble spots.

But such missiles also could be used against other targets, including civilian airliners, one reason for the U.S. and allied concern.

After the Soviet Union invaded Afghanistan, the CIA, with Saudi backing, provided sophisticated shoulder-fired Stinger missiles to Islamic militants seeking to oust Soviet troops.

The missiles played a significant role in the Soviets´ ultimate defeat in Afghanistan. But they also became a major headache for U.S. and western counter-terrorism agencies when anti-Soviet militants morphed into anti-Western militant factions including al Qaeda.

SAUDI ARABIA, QATAR ARMING REBELS

U.S. and allied officials acknowledge Syrian rebels have been receiving arms supplies from Saudi Arabia and the Gulf emirate of Qatar. But they said that the sophistication of the weapons being delivered had until recently been low.

An allied government source said it was clear wealthy individuals in Qatar and Saudi Arabia also were helping to finance anti-Assad groups.

The Saudis are on record calling for Assad´s ouster. Earlier this year, Prince Saud al-Faisal, the Saudi foreign minister, told an international conference that Assad´s "regime has lost its legitimacy and resembles an occupation authority...There is no way out of the crisis except through a transition of power, peacefully or forcibly."

In January, Qatar went even further when its ruler, Sheikh Hamad bin Khalifa al-Thani, told the CBS TV program 60 Minutes that Arab troops should be sent in to "stop the killing" by Assad´s forces.

A U.S. official who recently discussed the issue with Saudi and Qatari representatives said the weaponry now being shipped to Syrian rebels consists largely of small arms that would enable regime opponents to "protect their children." Deliveries to the rebels of MANPADS would represent a serious escalation.

US PROVIDING NON-LETHAL SUPPORT

Some prominent U.S. Republicans are urging a big step-up in U.S. aid for Assad´s opponents, including arms deliveries and even possible U.S. military involvement.

At a conference on Thursday hosted by the website Bloomberg Government, U.S. Senator John McCain suggested that the Obama administration´s cautious policy regarding the Syrian rebels was "shameful" and urged a major escalation in U.S. involvement.

"So what do we do? First of all, we stand up for them. Second of all, we get them weapons. Third of all, we establish a sanctuary with our allies - no boots on the ground, no boots on the ground - and use our and our allied air power to protect that zone and we help these people in a fair fight," McCain said.

At the same conference, however, Representative Mike Rogers, the Republican chairman of the House Intelligence Committee, warned: "We are just really not in a good position today to fully identify all of the groups, all of the factions, who´s winning that leadership fight," he said.

On Thursday, the New York Times reported that a small number of CIA officers had been deployed to southern Turkey, where they were helping U.S. allies decide which Syrian opposition elements should receive weapons deliveries.

The United States is understood to be supplying non-lethal support to Assad´s opponents, such as financing and communications gear, possibly including monitoring equipment. The Times said that the Obama administration has held back on providing rebels with intelligence information, such as satellite photographs, on the activities of Assad´s forces.

Riedel warned that Qatar authorities might not be too choosy about which Syrian rebels they are willing to supply with arms, though they would try to avoid giving them directly to al Qaeda.

"I don´t think that Qatar and the Saudis are as concerned as we are about surface-to-air missiles," Riedel added.

(Additional reporting by Phil Stewart, Tabassum Zakaria and David Alexander. Editing by Warren Strobel and Lisa Shumaker) (© Thomson Reuters 2012. 06/22/12)