BEIRUT — U.S.-trained rebels who recently returned to Syria said Wednesday they have lost contact with one of their officers and that they are investigating reports he defected and handed over his weapons to al-Qaida’s branch in the country.

The allegations come only days after the group of about 70 rebels returned to Syria after training in Turkey as part of the U.S. program to train and equip rebels to take part in the fight against the Islamic State group.

If confirmed, defection among the ranks of U.S.- trained rebels would be an embarrassment to the program, which has already been criticized as offering too little too late and failing to provide enough protection for those trained rebels once inside Syria. The selected rebels are said to undergo a thorough vetting process to ensure they focus on the fight against the IS.

U.S. officials have begun an overhaul of the efforts, including suggesting that the newly trained fighters operate as the New Syrian Forces alongside Syrian Kurds, Sunni Arab and other anti-Islamic State forces.

The U.S. Central Command confirmed Monday the graduates have re-entered Syria with their weapons and equipment and were to operate alongside existing western-allied forces.

Another previous batch of rebels trained by the U.S. had previously been hit hard by their rival, al-Qaida branch in Syria.

The first trained group, made up of 54 fighters, was wiped out by al-Qaida’s affiliate, the Jabhat al-Nusra or the Nusra Front, soon after it returned to Syria in July. In the attack, several members of the group were killed and others taken hostage while many fighters fled. U.S. Central Command spokesman, Air Force Col. Patrick Ryder, said that those rebels largely disbanded – of the 54, one was killed; one is being held captive; nine are back in the fight; 11 are available but not in Syria; 14 returned to Syria but quit the U.S. program and 18 are unaccounted for.

The new allegations, pertaining to the second group of U.S.-trained rebels, were first circulated on social media after the group returned to Syria last week. The U.S.-based SITE intelligence group, which monitors militant websites, said supporters of the Nusra Front first reported that a group of those newly trained by America have handed over their weapons to the militants after they were arrested. The report made no mention of a defecting officer.

Other supporters of the Nusra Front posted pictures of what they said were weapons seized from the U.S. trained rebels.

In response to the allegations, the U.S. trained group, known as Division 30, posted a statement on its Facebook page saying it is investigating.