Carter told the House Armed Services Committee that the US had initially envisioned training 24,000 Iraqi security forces at four sites by this fall.
But he said the US has received only enough recruits to be able to train about 7,000, in addition to about 2,000 counterterrorism service personnel.
"Our training efforts in Iraq have thus far been slowed by a lack of trainees. We simply haven't received enough recruits," Carter said at a hearing about US policy in the Middle East.
The White House announced last week that it was sending up to 450 more US troops to a new base in Anbar province of western Iraq, mainly to advise the Iraqis on planning and execution of a counteroffensive to retain Ramadi, the provincial capital.
Gen Martin Dempsey, chairman of the Joint Chiefs of Staff, has said that more such US hubs could be opened elsewhere in Iraq as the campaign advances.
Asked whether the 450 extra troops will make a difference in the fight against IS, Carter said the numbers are not as significant as the location, which is in the heart of Sunni territory.
"As I've told Iraqi leaders, while the United States is open to supporting Iraq more than we already are, we must see a greater commitment from all parts of the Iraqi government," Carter said, adding that the Iraqi leaders understand the need to empower a multi-sectarian Iraqi force as well as addressing organizational and leadership failures.
Carter testified along with Dempsey, who is finishing a four-year stint as chairman of the Joint Chiefs of Staff. Dempsey has expressed the view that the US needs to be patient and not give up on the Iraqi government's ability to fight IS.
