* Supplies handed to officers of the Free Syrian Army
* U.S. asks for proof aid going into intended hands
* U.S. relies on network of young Syrians for information
By Lesley Wroughton
WASHINGTON, July 25 (Reuters) - The United States has quietly been testing the Syrian opposition's ability to deliver food rations, medical kits and money to rebel-held areas as Washington prepares to send arms to the rebel fighters.
U.S. officials meet weekly in Turkey with Syrian opposition leaders to work out how best to keep supply lines open to rebel fighters and war-ravaged towns and districts.
One of the Syrian opposition's best-known female leaders, Suhair al-Atassi, attends the meetings as coordinator of the"non-lethal" aid that includes equipment for rebel fighters and local councils, as opposed to humanitarian aid for the displaced.
Supplies are handed to officers of the moderate Free Syrian Army (FSA) at clandestine locations that cannot be divulged for security reasons.
"I sign the paperwork, and shake the hands of the FSA official," said a U.S. State Department official involved in the effort. "I wish them well and walk away."
The rebels take aid for their own units and also distribute some of it to schools, clinics and local councils.
The United States has committed $250 million in non-lethalaid to Syria in addition to the $815 million in humanitarian assistance in support of the rebels fighting President Basharal-Assad's forces.
Recently, Washington began scaling up its assistance to bigger items like trucks, radios, large generators and sophisticated medical equipment.
Some of it is not only aimed at helping fighters but also at supporting civilian authorities in towns that have rejected Assad's rule.
"We are just now starting to send large equipment over the border for local councils and cities in liberated areas," the U.S. official said.
Syria's civil war has killed more than 100,000 people and forced millions to flee their homes. The involvement of Iran and its Lebanese ally Hezbollah in the conflict has shifted the balance of power on the battlefield in favor of Assad, increasing frustration among rebels over delays in the United States sending weapons to them.
With no U.S. diplomatic presence on the ground, Syria presents a unique challenge for aid coordinators.
U.S. officials say they rely on a network of some 75 young Syrians who collect information in rebel-held areas and report back to Atassi's unit. The information is often corroborated with U.N. groups.
The U.S. Congress cleared the way earlier this month for Washington to give the rebels not just non-lethal and humanitarian aid but also weapons. Lawmakers have only approved limited funding for the arms operation, as they fear that U.S. weapons and ammunition could end up in the hands of hardline Islamist militant groups.
"One of our main issues is to make sure that, whatever we do, that nothing gets in the hands of al Qaeda," said Dutch Ruppersberger, the top Democrat on the House of Representatives Intelligence Committee.
To keep track of the non-lethal aid already going into Syria, American officials ask the opposition to bring back photographic evidence of deliveries as proof that the goods made it into the right hands.
"If we are providing small amounts of cash to a local council to pay salaries we insist on signatures and photographs," said the official. "One of the ways to minimize the risk is we keep the amounts of cash small and would pay something like a stipend rather than a salary."
While it is not always easy to guarantee that supplies reach their intended recipients or that they don't eventually make their way to the black market, the Syrian opposition coordinators have begun to earn the trust of U.S. officials.
"They have so far passed the test," the official said.
France also sends supplies to the rebels, including envelopes stuffed with money handed over at the Turkish border.
(Reporting by Lesley Wroughton; Additional reporting byPatricia Zengerle; Editing by Alistair Bell and Eric Beech)