Afghan President Ashraf Ghani has said that since he took office in 2014 some 45,000 members of the country's security forces have been killed.
According to the SIGAR report, that amounts to nearly 850 soldiers per month.
The Afghan forces have also struggled to recruit and retain soldiers.
According to Reuters, the Afghan security forces have low levels of re-enlistment and also have issues with unauthorized absences.
The Afghan security forces have decreased to only 87% of their assigned strength.
According to SIGAR, this is the lowest level since the creation of the NATO mission in January 2015.
SIGAR also cited significant problems recruiting and retaining women in the Afghan Army.
According to the report, women in the Afghan National Army make up less than 2% of its assigned strength.