One of the 60 Syrian rebels trained and equipped by the United States has been killed, while at least five others have been captured, according to defense officials.

The death of the rebel came during an attack on Friday by about 50 militants in Northern Syria. U.S. officials believe Al Nusra, al Qaeda's affiliate in Syria, was behind the attack.

ADVERTISEMENT

At least five other rebels were captured after the attack, though it is unclear exactly when or how they were taken, officials said.

It's the first time that trained rebels — dubbed the New Syrian Forces — have suffered casualties.

In addition, more than a dozen other rebels with the group Division 30, who were vetted but not trained and equipped by the United States, have also been wounded and captured.

Five Division 30 rebels were captured on Wednesday before the attack, and eight of them wounded during Friday's attack, bringing the total number of those killed or captured to at least 19.

Officials said not all of the 60 rebels who have been trained by the United States were at the site of Friday's attack, but declined to say where they were. They also said about 30 militants were killed in the attack, which five U.S airstrikes helped repel.

While the U.S. has decided to protect the rebels against attacks by any outside forces, it's unclear whether the U.S. will seek to recover the captured rebels, one official said.

The early targeting of the rebels raises questions as to whether the U.S. will be able to field an effective force, with a tenth of them taken out within weeks of their deployment.

Another batch is being trained, but it is unclear how many or when they will be deployed into Syria.

The program has gotten off to a slow start, and lawmakers were taken aback last month when Defense Secretary Ashton Carter revealed that only 60 rebels have made it through the program, which was created to battle the Islamic State in Iraq and Syria (ISIS).

Defense officials have said they still hope to train 3,000 rebels by the end of the year.

Officials say the plan is to continue training the rebels until a division is fielded. The Syria train and equip program envisions fielding 15,000 rebels over three years -- ostensibly one division.

A diplomatic official close to the opposition said the trained rebels will be embedded within vetted groups, in order to maximize the effect they will have in the battlefield.

A characterization of the program by Air Force Col. Pat Ryder, chief spokesman for U.S. Central Command, reinforced that idea.

"As has been mentioned previously, we are only accepting candidates from recognized groups, so vetted Syrian fighters who are selected arrive as members of a group already, Ryder said.

"A key aspect of the Syria Train and Equip program is to train these personnel as units to effectively fight together, which includes training and developing their leaders and ensuring they have a chain of command that can lead forces effectively and appropriately," he added.

"Ultimately, this will enhance the effectiveness of these coalition-trained Syrian forces as they return to Syria as units to fight ISIL and defend the Syrian people."