WASHINGTON — The Pentagon said Friday that an American-trained rebel commander in Syria had surrendered trucks and ammunition this week to forces affiliated with an offshoot of Al Qaeda.

Military officials, who days before had denied reports of the handover, called the disclosure “very concerning” and a violation of guidelines for the American training program for Syrian rebels.

United States Central Command said the rebel commander told the military that he surrendered the American-provided ammunition and trucks early this week in exchange for safe passage through an area in which the Qaeda offshoot, the Nusra Front, operates.

The matériel “equated to roughly 25 percent” of the equipment issued to the rebel group led by the commander, who had recently completed the training program, said Col. Patrick Ryder, a spokesman for Central Command, or Centcom.

The news came as Centcom was reeling from a series of alarming reports about the Syria training program, which has come under increasing criticism from Congress.

Last week, Gen. Lloyd J. Austin III, the head of Centcom, told a Senate committee that only four or five Syrians trained by the American military to confront the Islamic State remained in the fight — an acknowledgment that a $500 million program to raise an army of Syrian fighters had gone nowhere. General Austin also said that the United States would not reach its goal of training 5,000 Syrian fighters anytime soon.

Colonel Ryder said that Centcom was notified of the equipment handover at 1 p.m. on Friday. Earlier this week, the Nusra Front posted an image on Twitter of a rifle issued by the American-led coalition and announced that Syrian rebels trained by the United States had handed over their weapons as they re-entered Syria.

Centcom said Friday that its earlier denial of that claim had been based on statements by Syrian rebels that all personnel and equipment remained under their control. Military officials had also said that the image on Twitter was an old picture taken from the Facebook page of a rebel.

“In light of this new information, we wanted to ensure the public was informed as quickly as possible about the facts as we know them at this time,” Colonel Ryder said. “We are using all means at our disposal to look into what exactly happened and determine the appropriate response.”