President Barack Obama's administration vowed to press ahead with a half-billion-dollar mission to train Syrian opposition fighters Tuesday, despite fears that its initial failures have dented US credibility.

After the routing of a 54-strong US-trained force by Al-Qaeda allied rebels, Pentagon spokeswoman Commander Elissa Smith told AFP there was no plan to pause or scale back the program.

"We are continuing with training," she said, "despite the initial challenges, (the Department of Defense) remains firmly committed to building the capacity of the moderate Syrian opposition."