(CNN) The Pentagon is poised to begin, as soon as this week, a controversial program to train moderate Syrian rebels at locations in Turkey and Jordan to fight ISIS, according to several U.S. defense officials.

The first of 400 U.S. military trainers have now arrived in both countries. Out of a list of 3,000 rebels who have expressed interest in joining the program, some 400 have passed the initial security screening. Once final approval is given, they will be training on small arms, radios, medical gear and battlefield tactics.

The plan is for them to return to their towns and villages and be able to specifically defend them against ISIS. However, U.S. officials are aware, they say, of the risk that some of the trained fighters could decide to take their weapons and fight the Assad regime which is not the goal of the training.

JUST WATCHED Liberation from ISIS comes at a cost Replay More Videos ... MUST WATCH Liberation from ISIS comes at a cost 02:19

The U.S. plans to provide them with weapons, trucks and tactical radios so they can communicate among themselves on the ground. No final decision has been made about any U.S. provided protection -- such as airstrikes against ISIS or the Assad regime -- for the rebels when they return home.

Read More