The Britain-based Syrian Observatory for Human Rights said earlier that Al-Nusra Front had kidnapped eight members of a US-backed rebel unit.
But Commander Elissa Smith, a Pentagon spokesman, denied that any of those captured were among the fighters who have passed through a Pentagon program to prepare them to fight the Islamic State jihadist group.
"While we will not disclose the names of specific groups involved with the Syria Train and Equip Program, I can confirm that there have been no New Syrian Force personnel captured or detained," she told AFP.
The first batch of graduates of the USD 500 million program, reportedly 54-strong, are said to have recently crossed into Syria from Turkey.