Videos have appeared on the internet showing Syrian rebels using what appear to be US anti-tank rockets, the first significant American-built armaments in the country's civil war.
They would signal a further widening of the conflict, with rockets from Russia and drones from Iran also used by forces of President Bashar al-Assad.
None of that equipment, however, is seen as enough to be decisive in a stalemated war, with Assad dominant in 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 anti-tank rockets shown in the videos. Some analysts suggested they might b e from a state such as Saudi Arabia, a United States ally, probably with Washington's approval.
US officials declined to discuss the rockets, which appeared in Syria around the same time it was 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 backing for the insurgency, given the widely dominant role played by Islamist militants. A proposal to supply surface-to-air missiles was considered but rejected.
National Security Council spokeswoman Bernadette Meehan said the administration was giving support she did not define.
"The US 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.
"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 remained small, reports of their presence were spreading, analysts said.
"With US-made 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, of 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. Two have been removed, but one remains on YouTube.
The rebel faction shown operating the US missiles in the first videos, is the relatively secular and moderate Harakat Hazm, which declined comment. But an opposition activist based in 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.
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.