The United States said on Tuesday it had indications that Syrian rebels trained by the US military were captured by fighters from al Qaeda's Syria wing, al Nusra Front, in the latest blow to a fledgling programme at the centre of America's war strategy.
The Pentagon said in a statement it was monitoring the situation but had "no further details to provide".
A US defence official, speaking to Reuters on condition of anonymity, said at least five Syrian rebels were believed to have been captured.
That followed an attack by the Nusra Front on Friday believed to have killed one of the so-called "New Syrian Forces", in what would be their first battlefield casualty.
The incidents underscore the extreme vulnerability of the New Syrian Forces, a still tiny group thought to number less than 60 who were only deployed to the battlefield in recent weeks.
The White House, Pentagon and State Department this week restated US' commitment to defend them, including with airstrikes. It launched five strikes to help the New Syrian Forces eventually repel Friday's attack, for example.
The New Syrian Forces are at the core of US President Barack Obama's strategy to build them into a force large enough to wrestle territory captured by the Islamic State, instead of injecting American combat troops into Syria's messy civil war.
The Islamic State is thought to control around a third of the territory in Syria.
But instead of Islamic State, the New Syrian Forces appear to be consumed with a battle against Nusra Front.
One of the most powerful insurgent groups in northern Syria, the Nusra Front has a record of crushing rebel groups that have received support from western states, including the Hazzm movement that collapsed earlier this year.
Pentagon spokesperson Captain Jeff Davis said the United States had not lost focus of the threat from the militant group.
"The al Nusra Front has publicly declared itself an al Qaeda affiliate ... and we remain committed to working with our partners in the region to counter the threat," Davis told a news briefing.
The US military's training programme has been challenged from the start, with many candidates being declared ineligible and some even dropping out, putting the Pentagon far behind on its goals to train around 5,000 fighters a year.
Obama's requirement said they target Islamic State militants has sidelined huge segments of the Syrian opposition, which is focussing instead on battling Syrian government forces.
The Pentagon on Tuesday also offered additional details on Friday's attack, saying it believed "several members" of the US-trained force were attacked by about 50 fighters believed to be from al Nusra Front. It declined to confirm any casualties.