Defense Secretary Jim Mattis placed the blame for the current mess in Afghanistan squarely on the Obama administration, telling Congress Wednesday that by cutting support for the Afghan forces prematurely, President Obama allowed the Taliban to regroup and recover.

"I believe that we pulled out forces at a time, as you know, when the violence was lower, but we pulled them out on a timeline rather than consistent with the maturation of the government and the security forces," Mattis told a Senate subcommittee Wednesday.

"The result was that as security declined, all the other stresses have come to bear, to include heavy casualties on the part of the Afghan forces, other nations pulled their forces out as well, and the Taliban was emboldened."

Mattis said among Obama's worst mistakes was his decision to declare combat operations over at the end 2014, and subsequently withdraw U.S. combat firepower, in particular air support, from the Afghan military, just when it needed it most.

"At one point, when we reduced our forces there, I believe in what was probably in hindsight a misguided application of our forces, we restricted them from using our air support, with some idea we would wean them off the need for it," Mattis said. "That meant in the mountain country these troops were often fighting at a disadvantage."

Mattis said his new strategy, which will be finished in the coming weeks, will restore the "high ground" by restoring NATO air forces to a position of dominance

Mattis, who now has been granted full authority to decide troops level in Afghanistan, said he is a developing a "refreshed, new approach," before deciding on how to augment the 8,400 U.S. troops.

Pentagon sources say Mattis is considering adding 3,000 to 5,000 military trainers and advisers to the NATO Resolute Support mission.