KABUL, Afghanistan?Four American soldiers were killed Monday in what Nato said was believed to be a mid-air collision between two helicopters in southern Afghanistan.
Two other soldiers were injured, Nato?s International Security Assistance Force (Isaf) said in a statement.
"Four Isaf service members were killed and two others injured in the incident," it said.
"The incident is currently being investigated, but it is confirmed that hostile fire was not involved," it said.
The south is the most volatile part of Afghanistan for the more than 100,000 Nato (North Atlantic Treaty Organization) and United States troops fighting a resurgent Taliban.
The independent icasualties.org website, which keeps a running tally of foreign troops? deaths in Afghanistan says that so far in 2009, 424 have died.
