The commander of the US army reserve says it is rapidly degenerating into a "broken" force.
Part-time soldiers form 40% of US troops in Iraq
Lt Gen James Helmly, in a leaked memo to the Pentagon, says the reserve has reached a point where it cannot fulfil its missions in Iraq and Afghanistan.
Reservists provide a large share of US troops in Iraq. The army says Gen Helmly's concerns are being addressed.
But analysts say they will fuel criticism that Pentagon policies are harming the US all-volunteer military.
The army reserve is a force of about 200,000 part-time soldiers who chose not to sign-up for active duty but can be mobilised in time of need.
Together with National Guard troops - who also serve part-time - reservists account for about 40% of US troops in Iraq.
The internal memorandum was first reported on Wednesday by the Baltimore Sun newspaper, and appeared on the internet later in the day.
In it, Gen Helmly says that under current procedures his forces will be unable "to meet mission requirements associated with Operation Iraqi Freedom and Enduring Freedom" in Afghanistan.
Gen Helmly takes issue with a number of "dysfunctional" Pentagon practices, including:
- Financial incentives to attract and retain reservists on active duty, which the general says confuses "volunteers" with "mercenaries"
Reservists being called to active duty at only a few days' notice
- Reserve troops being required to leave equipment for other forces after serving in Iraq and Afghanistan.
The BBC's Nick Childs in Washington says such blunt words from a senior general are likely to provide further ammunition for critics who argue that the current policies are doing long-term harm to America's all-volunteer military.
US armed forces have been placed under considerable strain by the wars launched by President George W Bush during his first term.
Some commentators have accused Defence Secretary Donald Rumsfeld of not accepting that huge troop deployments were needed, notably in Iraq.
Reacting to the leaked memo, Senator Jack Reed, a Democrat, told the Associated Press news agency: "By consistently underestimating the number of troops
necessary for the successful occupation of Iraq, the administration has placed a tremendous burden on the Army Reserve and created this crisis."
In its response, the army said it acknowledged that changes have to be made in the way reserves are used and mobilised to deal with the new threats.