An explosion hit foreign troops on a foot patrol in southern Afghanistan killing three service members, the US-led coalition said Thursday. The last three months have been the deadliest for international troops in Afghanistan since the 2001 US led invasion. A record number of US and NATO troops are in Afghanistan - around 65,000 - exposing more soldiers than ever to increasingly lethal Taliban bombings and ambushes. The coalition did not release any details about Wednesday's attack, including the troops' nationalities or the blast's location. American forces comprise the vast majority of the US-led coalition, which includes special forces units and soldiers who train Afghan army and police. The 40-nation NATO-led force operates under a separate command.