* Surface-to-air missiles could be stolen or resold
* U.S. "held back" on weapons supplies to rebel forces
* Experts less worried about nuclear, chemical stockpiles
By Mark Hosenball and Andrew Quinn
WASHINGTON, Aug 24 (Reuters) - The U.S. and NATO are deeply concerned about possible looting and resale of conventional weapons, particularly shoulder-fired anti-aircraft missiles, from Libyan arsenals as Muammar Gaddafi's rule crumbles.
Some Western officials say there is scattered intelligence indicating that weapons stolen from Gaddafi's stockpiles may have made their way to insurgents or militants in nearby countries, such as Niger, and North African nations.
The officials said they were much more worried about leakage of conventional weapons from Libyan arsenals than they were about possible theft or misuse of Libya's residual supplies of radioactive materials and chemical agents.
Of particular concern is the possibility that some of Gaddafi's surface-to-air missiles, also known as MANPADs (Man-Portable Air Defense Systems), which could be used to attack airliners, might fall into the hands of militants.
"They are very dangerous, unstable weapons, and if they fall into the wrong hands, they can be extremely disruptive, as we've seen around the world," Victoria Nuland, a State Department spokeswoman, said on Wednesday.
The current status of Gaddafi's arms caches is unclear, but officials in Washington believe it is likely many have been overrun and taken over by anti-Gaddafi forces.
Gaddafi, however, is believed to have over-stocked his forces with far more relatively-sophisticated equipment than they would ever use, including weapons which could pose serious threats if they fell into the hands of militants.
Nuland said it was well known that Libya was "awash in weapons" and that the U.S. was worried about their proliferation.
The U.S. government has sent two teams to the region, including to Libya's neighbors, to look at the MANPADs issue, she said.
"I wish all good to the rebels but there are very, very serious concerns as to where Muammar Gaddafi's weapons end up," said Douglas Frantz, a former senior official of the U.S. Senate Foreign Relations committee who is now with international investigations firm Kroll Inc.
Frantz said that fear of theft was one reason why the Obama administration "held back" from supplying heavy weaponry to anti-Gaddafi forces.
In addition to MANPADs, weaponry held in Gaddafi's stockpiles that could be at risk is believed to include, anti-tank rockets, armored vehicles, rocket-propelled grenades and explosives.
Experts said that most of Gaddafi's surface-to-air missiles are thought to be SA-7s, which are not particularly accurate and are most effective at bringing down aircraft when fired in a barrage. But Bruce Riedel, a former CIA expert on the region, said it is possible Libyan arsenals also hold more advanced MANPADs.
Any such weapons, Riedel said, are portable, easy to use and a "real threat to aviation in Europe and Arab countries."
In November 2002, al Qaeda-linked militants launched two shoulder-fired missiles at a passenger jet from Israel's El Al Airlines after it took off from Mombasa, Kenya, nearly bringing it down.
On Monday, congressman Mike Rogers, chairman of the House of Representatives Intelligence Committee, said: "We must ensure that Gaddafi's stockpiles of advanced weapons, chemical weapons and explosives don't fall into the wrong hands."
Many U.S. and allied experts say they are less worried about theft or misuse of Libya's remaining supplies of chemical agents and radioactive materials.
A United Nations expert familiar with Libya's remaining stockpile of mustard gas said that years ago Libya gave up equipment and material needed to turn the gas into weapons and that the gas itself has likely decayed so extensively that it presented a more serious environmental than military threat.
Olli Heinonen, a former senior official of the United Nations' atomic energy watchdog, said on Wednesday that stockpiles of low-enriched uranium and other radioactive material were still at a research facility near Tripoli which could be used to make a "dirty bomb" -- a weapon that experts say probably would not kill many people but would cause massive disruptions.
Nuland said the United States had been monitoring Libya's known missile and chemical agent storage facilities since the start of the conflict.
"We believe that these known missile and chemical agent storage facilities remain secure, and we've not seen any activity, based on our national technical means, to give us concern that they have been compromised. But that monitoring will continue," she added.
Editing by Paul Simao