Basic leadership failures in many Afghan police and military units are helping drive casualty rates to ever-higher levels, the top US and NATO commander in Afghanistan warned Sunday.

"We are very concerned about the casualty rate ... This year has been the same, or slightly higher (than last year), depending on the unit and region," US Army General John Nicholson told reporters.

"One of the principal factors for the high casualties has been the leadership, the failures of leadership at certain levels. Primarily this has been in the police and to a lesser extent the army."