A US government report blames Iraq for failure in reconstruction works.
A senior Pentagon official has told The Washington Post newspaper that some weapons were probably being used against US troops.
He said that an Iraqi brigade, created in Fallujah, had disintegrated in 2004 and had begun fighting American soldiers.
Since 2003, the US has spent about $19.2bn to develop Iraqi security forces.
The US defence department has recently asked for another $2bn to continue the train-and-equip programme.
According to the GAO, the government had funded the programme for Iraqi security forces outside traditional security assistance programmes, providing the Pentagon with a degree of flexibility in managing the effort.