By delivering light weapons to Syria's rebels, the United States is seeking to pile pressure on Damascus and its patrons, rather than tip the balance on the battlefield, experts said Thursday.

Three months since President Barack Obama promised military support to opposition forces, the Washington Post reported that the first deliveries of guns and ammunition began arriving two weeks ago, after an alleged chemical weapons attack that Western governments blame on the Syrian regime.

The leak came only hours before high-stakes talks in Geneva between top American and Russian diplomats on a proposal designed to avert possible US military action against President Bashar al-Assad's forces.

"I think the timing of the leak is more significant perhaps than the actual weapons going in at the moment," said Charles Lister, an analyst at IHS Jane's, a defense consultancy.

The arms shipments reportedly do not include anti-tank or anti-aircraft weapons coveted by the rebels, and as result, the deliveries will "have a relatively small impact in terms of the conflict itself," Lister said.

"But within the political dynamics, it might have sent much more of a significant message to the international community, to President Assad, and to his supporters like the Russian government," he told AFP.

The CIA, which oversees the secret operation to arm the rebels, declined to confirm or deny the reports.

Salim Idriss, the former Syrian military general who now leads the rebel Free Syrian Army, said no guns delivered by the United States had reached his fighters on the ground.

"We didn't receive any kind of lethal material from our American friends," Idriss told National Public Radio.

Instead, the rebels had received food, medical supplies, bullet proof jackets, light vehicles, ambulances, night-vision googles and "communication equipment," he said.

The rebel commander's comments underscored growing tensions between opposition forces and the US administration, which has been criticized by rebel fighters and their Arab allies for failing to do more to support the more moderate elements of the opposition.

President Barack Obama's decision to put off any punitive missile strikes against Syria over its alleged use of sarin gas, pending the outcome of a Russian proposal to impose international control over Assad's banned chemical weapons, has also fueled frustration among the opposition, analysts said.

The rebels and their advocates have long demanded Washington deliver anti-tank and anti-aircraft shoulder-launched weapons, which they say could turn the tide in the two-year-old civil war.

"If only light weapons are delivered, then it will only help the rebels in a light way," said Christopher Hammer, a senior naval analyst at the Institute for the Study of War, a Washington think tank.

"Without delivering heavier, consequential weapons, the balance of power essentially remains unchanged."

While the White House maintains a majority of the opposition are "moderates," officials have worried that heavy weapons could fall into the hands of extremists with links to Al-Qaeda.

Saudi Arabia and Qatar reportedly already have begun supplying the rebels with some heavier weapons, and US officials privately have not ruled out delivering more effective weapons in the future.

The initial US arms shipments could serve as a "test," to see if the weapons end up with the rebels Washington favors and that supply routes are sufficiently secure, according to Lister.

In another apparent bid by Washington to ratchet up pressure on the Syrian regime and its Iranian and Russian backers, US officials have held out the possibility of expanding support to the rebels by shifting the effort to the Pentagon from the CIA.

Officials told AFP last week the idea was under serious consideration, and would result in a larger scale program to supply weapons and vehicles to the rebels.

At a Senate hearing last week, top US military officer Martin Dempsey said he was ready to discuss how to help the moderate opposition "in a more overt way," suggesting the Pentagon might take charge of the initiative.

The administration's calculus on aiding the rebels has clearly shifted since last year, when the White House rejected a proposal to arm the opposition despite support from the then heads of the Pentagon, the State Department and the Central Intelligence Agency.