U.S. Central Command said the Syrian rebels, known as the New Syrian Forces and who are trained by the U.S., surrendered the equipment to an Al-Nusra Front group in return for Al-Nusra granting "safe passage."

"If accurate, the report of NSF members providing equipment to Al-Nusra Front is very concerning and a violation of Syria train and equip program guidelines," said Col. Patrick Ryder, a U.S. Central Command spokesperson.

The trucks and ammunition amount to 25% of the NSF's coalition-issued equipment.

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Central Command originally denied that the equipment had made its way to the jihadist group. Al-Nusra tweeted a photo of a rifle issued by U.S. coalition forces this week, but Central Command had determined that the picture was fake.

But now the U.S. is learning from the rebels that six trucks and some of their ammunition was commandeered by Al-Nusra.

"We are using all means at our disposal to look into what exactly happened and determine the appropriate response," Ryder said.

Western forces have struggled to stymie the threat posed by Islamic forces in Syria since the country's deadly civil war began in 2011.

CNN's Barbara Starr contributed to this report.