In Afghanistan, four US soldiers were killed and two others were injured during a mid-air collision between two International Security Assistance Force (ISAF) helicopters in south of the country Monday morning. <br/> <br/>An ISAF statement said the incident is currently being investigated, but ruled out hostile fire. In another incident, twelve militants were killed in a joint operation by international security forces in western Afghanistan. <br/> <br/>The statement said that as the joint force was departing the area, one helicopter went down due to unconfirmed reasons. There are reports of casualties among soldiers and a recovery operation is underway. <br/> <br/>The current year is proving to be the deadliest for ISAF forces since 2001 after removal of Taliban from power. <br/><br/>