The withdrawal of surge troops, which began in July, follows an unprecedented number of Western soldiers being shot dead by their Afghan colleagues -- 51 so far this year - and as anti-West protests sweep Muslim countries.
There are still some 68,000 US military forces in Afghanistan, as well as some 40,000 from NATO's ISAF coalition.
The US-led effort to contain the Taliban insurgency involves a phased withdrawal of Western troops, and training Afghan forces to take their place. The plan is for Afghans to take charge of their own security by the end of 2014.
The temporary increase in troops helped Western-led forces regain ground in Afghanistan's south and southwest against the Taliban, which has now stepped up attacks in the eastern region bordering Pakistan.
Decisions on the pace of future withdrawals in line with the 2014 deadline are pending.
On Tuesday, NATO announced that it would limit joint operations with Afghan forces, marking a setback to the US-led strategy for containing an 11-year Taliban insurgency.
Australian Brigadier-General Roger Noble, deputy to ISAF's operations chief, said the string of insider attacks had been dispiriting.
Mr Noble on Thursday said the decision to scale back joint operations was "just normal military business and common sense," and insisted it would not derail the drawdown effort.
"The campaign remains on track to achieve its objectives," Mr Noble told Pentagon reporters from Kabul.