The move is not seen as enough to change course of war for now
Online videos show Syrian rebels using what appear to be US 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.
US 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.
US 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 US rockets seen remains small, reports of their presence are steadily spreading, analysts say.
"With US-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 US 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 US arms had figured in Syria's fighting.
His information could not be confirmed independently.
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.