The comment, made in an exclusive interview with CNN, was the harshest public criticism of the Iraqi security forces to date from the Obama administration. The United States has said local fighters, rather than U.S. forces, must lead the fight against ISIS, a strategy that has come under withering criticism as the terror group gains ground in Iraq and Syria.
The remark surprised Iraq's Prime Minister Haider al-Abadi, who told the BBC that Carter was "fed the wrong information."
In a Monday phone call with Abadi, Vice President Joe Biden "recognized the enormous sacrifice and bravery of Iraqi forces over the past 18 months in Ramadi and elsewhere," according to a statement from the White House.
Biden, who told Abadi before Ramadi's fall that shipments of weapons were being expedited to help protect the city, explained to the prime minister on Monday the U.S. was planning to ramp up training to combat ISIS truck bombs, which were deployed in brutal fashion during the group's takeover of the Anbar capital.
Meanwhile, U.S. officials were parsing what precisely Carter meant when he told CNN Pentagon correspondent Barbara Starr that "we have an issue with the will of the Iraqis to fight ISIL and defend themselves," despite outnumbering ISIS forces.
A senior administration official said Carter's remarks were in reference to the Ramadi siege specifically, which came after months of fighting and was hastened by a rash of ISIS suicide bombings, some of them at the same magnitude as the 1995 Oklahoma City blast.
"The reference to lack of will was in relation to this specific episode, which followed 18 months of fierce (Iraqi Security Forces) attrition against ISIL in Ramadi, coupled with what the Iraqi government has acknowledged were breakdowns in military command, planning, and reinforcement," the official said.
A senior defense official pointed to specific factors that may have contributed to Iraqi troops' lack of fighting will in Ramadi, including the absence of regular payments, the inability to visit family members and a general sense that commanders weren't looking after their battalions.
According to this official, the U.S. has grown increasingly concerned about a lack of leadership skills within the Iraqi ranks, seen as crucial to winning the support of troops in combat situations like the battle for Ramadi.
The White House has consistently ruled out sending American combat forces back into Iraq after the decade-long war begun by President George W. Bush. Instead, the U.S. is relying on a strategy of empowering local forces to beat back ISIS where they've made gains.
President Barack Obama, speaking to The Atlantic magazine
last week, said that "if the Iraqis themselves are not willing or capable to arrive at the political accommodations necessary to govern, if they are not willing to fight for the security of their country, we cannot do that for them."
Officials say in Anbar province, the equipping and training of Sunni tribes is a priority as Iraqi forces regroup and attempt to retake Ramadi.
"The rapid integration of the Sunni tribes into the fight alongside other Iraqi forces is essential as they will be the most invested in fighting for their areas," an administration official said.
Michele Flournoy, a former undersecretary of defense who Obama considered naming to the top Pentagon post, said on CNN Sunday the administration has "under-resourced" its counter-ISIS strategy.
"We need to provide more fire power support, more intelligence surveillance," she told CNN's Jim Acosta on "State of the Union."