Nato's International Security Assistance Force did not immediately have any details on the incident.
 
Despite a large Nato presence, much of Afghanistan is being swept by anarchy and lawlessness.
 
The Taliban have apparently regrouped after being driven out of Kabul by US-led forces in November 2001 and have since stepped up attacks on Nato and Afghan forces.
 
A resurgent Taliban have also threatened a renewed offensive in coming months.