The department Friday published a 119-page report to Congress on progress made in Afghanistan that said there were 344,602 members of the Afghan national security force as of August, nearly twice the number in its ranks in 2009.
The report said, however, the force is struggling to develop as a comprehensive fighting force.
"The [force] will be challenged to close a number of capability gaps in areas where ISAF currently provides support," the report said. "Afghanistan's security ministries, despite substantial progress, still require substantive improvements in planning, programming, budgeting, and acquisition."
International forces operating in Afghanistan wrap up their combat obligations next year.
Defense Department spokesman George Little said Afghan forces are leading in nearly all of the combat operations in the country.
"Afghan security forces are now successfully providing security for their own people, fighting their own battles, and holding their own against the insurgency," he said in a statement.
The report said Afghan forces face problems associated with corruption, and most service personnel have trouble reading.
"Beyond 2014, the [force] will still require substantial ... mentoring, as well as financial support, to address ongoing shortcomings," the report said.