KABUL -- Coalition forces said that four American soldiers were killed in Afghanistan on Monday when two NATO helicopters collided in mid-air. According to NATO-led International Security Assistance Force (ISAF), the accident took place in southern Afghanistan. Aside from the casualties, there were also two people who got injured in the incident. The coalition released a statement saying that the incident is currently under investigation, but it was verified that enemy fire was not in any way involved in the accident. In a separate report, another helicopter crashed in the western region of the country during an offensive against Taliban militants. NATO soldiers added that over a dozen Taliban fighters were killed in the firefight that followed. A NATO statement said that the coalition suffered casualties but a recovery operation is ongoing.