The US has questioned Iraq’s commitment to fighting Islamic State and lamented the lack of Iraqi recruits available for training.
“We must see a greater commitment from all parts of the Iraqi government,” said US Defence Secretary Ashton Carter, speaking in Washington.
US trainers have only received 7,000 recruits in recent months, with a target of 24,000 by autumn.
“We simply haven’t received enough recruits,” said Mr Carter.
The top US military official also highlighted the need to empower Sunni tribesmen in the fight against the Islamic State.
“Our efforts need to reinforce inclusivity and multi-sectarianism,” he said.
US officials are pressuring the Iraqi government to be more inclusive and representative of Iraq’s three major ethnic groups – Sunnis, Shias and Kurds.
Mr Carter was speaking at a hearing held by the House Armed Services Committee.
For several months, the US has been providing training and advice to Iraqi forces as they try to push back Islamic State fighters who have gained territory in Iraq.
However, US troops are so far not engaging in active combat themselves.
Many Republicans in Congress say the US should not rely solely on the Iraqi forces.
“There is a sense that we are at a particularly perilous time and that US policy and strategy are inadequate,” said the committee’s chairman, Mac Thornberry, a Republican from Texas.
Separately, Mr Carter also cited similar problems with recruitment in the US’s programme to train Syrian opposition forces.
“We have enough training sites and so forth. For now, we don’t have enough trainees to fill them,” he said.