The US government has suspended delivery of non-lethal aid to Syria's rebels because an Islamist group has taken control of the warehouses where the aid was stored.
US authorities said the seizure exposed the rapidly shrinking authority of the moderate rebel factions backed by America and its Western allies.
The United States continues to supply humanitarian aid to Syrian civilians impacted by the long-running civil war, and officials indicated that a covert programme to provide small arms and ammunition to rebels in the southern part of the country was ongoing.
But the suspension of the supply of food, medical kits, trucks and communications equipment to rebels in the north illustrated the erosion of US influence over the groups battling to topple Syrian President Bashar al-Assad.
A growing number of factions, disillusioned by the low level of Western support, have been aligning themselves with Islamist groups that receive more generous funding from Gulf Arab states.
The recently created Islamic Front, which seized control of the warehouses, includes the biggest Islamist factions in Syria.
The front's expanding influence, and the fact that it is not affiliated with al Qaeda, may leave the US with little choice but to work with it, if Washington is to have any hope of retaining influence over Syria's opposition, analysts and rebel leaders say.
The cluster of warehouses, in the border town of Atmeh, was controlled by the Supreme Military Council, which was tapped last June as the chief recipient of US aid.
More important than the relatively meager contents of the warehouses was the international legitimacy they conferred on the SMC and its leader, General Salim Idriss, here, ahead of crucial peace talks due to begin in Geneva in January.
On Friday, the Islamic Front told fighters from the SMC that an al Qaeda-linked group was preparing an attack on the warehouses, and offered to help defend them.
The Islamic Front fighters then ejected the SMC rebels at gunpoint. It is unclear whether there ever was a threat from al Qaeda, and some rebel groups have charged that the warning was a ploy.
The warehouses contained food, including military MREs, medical kits, communications equipment and several pick-up trucks, typical of the items the United States is supplying under a US$16.9 million (NZ$20m) programme, US and rebel officials said.
The buildings also contained sizeable quantities of small-arms ammunition from an unknown source, according to an opposition figure close to the rebels who asked not to be named because the subject is sensitive.
"We're obviously concerned," about the warehouse takeover, State Department spokeswoman Jennifer Psaki said.
"We're not prepared at this point to make a broad statement about what it means and what the long-term impact will be. We're in close contact with the SMC. And we will see over the course of time what this means."
The non-lethal assistance promised last spring by the Obama administration has not made a significant difference in the fighting capacity of the rebels, which rebel leaders say may be one of the reasons why many fighters grew disillusioned with the SMC.
Promised items such as body armour and night vision goggles never materialised. Deliveries were disrupted by the takeover of territory along the northern border by the al Qaeda affiliated Islamic State of Iraq and Syria, which Washington has labelled a terrorist group.
The Islamic Front - which includes many Salafi Islamists - has been pressing for inclusion in the SMC, out of concern that it won't have any input at the talks in Geneva, according to rebel commanders involved in the discussions.
The talks will be the first between Assad's government and the opposition since the conflict erupted nearly three years ago.
Louay al-Mokdad, an SMC spokesman, criticised the aid suspension as premature, saying he expected negotiations between the Islamic Front and the SMC to resolve the dispute over the warehouses soon.
"It was just a misunderstanding between brothers," Mokdad said.
"The American administration decision was a little bit rushed. We understand their concerns but we hope they will rethink this decision."
He said the council would welcome some form of merger with the Islamist factions.
"We are open to everyone," he said.
"It's time to be united all against the Syrian regime."
But the prospect of rebel unity is complicated by US concerns about the relationships between some of the Islamic Front's factions and sanctioned terrorist affiliates such as ISIS and also the somewhat less extreme Jabhat al-Nusra.
US officials have had held meetings with Islamic Front leaders in recent weeks to explore their views, rebel commanders and US officials say.
The US government now has to decide whether it wants to maintain some influence over Syria's rebels, which means dealing with the non-al Qaeda Islamists, or to surrender the battlefield to increasingly extremist Islamists over which it has no control.
"If we keep these groups at arm's length, what influence can we have?" said Andrew Tabler of the Washington Institute for Near East Policy. But, he added, "talking to them is one thing. Working with them is another."
- Washington Post