DAMASCUS — The United States said on Tuesday it had indications that Syrian rebels trained by the U.S. military were captured by fighters from al-Qaeda’s Syria wing, Nusra Front, in the latest blow to a fledgling program at the center of America’s war strategy.

The Pentagon said in a statement it was monitoring the situation but had “no further details to provide.”

A U.S. defense 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 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 only deployed to the battlefield in recent weeks.

The White House, Pentagon and State Department this week restated the U.S. 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 President Obama’s strategy to build them into a force large enough to wrest territory from ISIS, instead of injecting American combat troops into Syria’s messy civil war.

One of the most powerful insurgent groups in northern Syria, 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.