The Nusra Front militants released seven rebels of the so-called 30th Division, but kept the group's commander, Colonel Nadim al-Hasan, in custody, the Syrian Observatory for Human Rights cited a statement by the Division 30 group as saying.
Last July, the rebels' 30th Division, part of the US-backed Free Syrian Army (FSA), said their leader and his comrades were snatched by the Nusra Front in the northern countryside of Syria's northern province of Aleppo, according the statement.
The U.S.-trained group said it calls on the "brothers in the Nusra Front" to release the colonel along with his comrades to "prevent the bloodletting among the Muslims and to preserve the unity of line in the face of side disputes."
In June, U.S. official said the first unit of the FSA, which was trained by the United States to fight the so-called "Islamic State" (ISIS) group, had crossed into Syria from the training camps in neighboring Jordan, according to the Washington Post.
The fighters, numbering about several dozen, are among the first to be trained by the United States, which announced it was going to train "vetted" and "moderate" rebels to fight the ISIS.
In its statement Sunday, the rebel group expressed gratitude for the release of the seven rebels, urging the Nusra Front to release the commander and others kidnapped alongside him.