WASHINGTON (Reuters) - An explosion at an entry control point at Bagram airfield near the Afghan capital Kabul caused a small number of casualties, the U.S. military said on Wednesday.  An explosion occurred outside an entry control point at Bagram Airfield at 5:38 p.m. local time today,  a statement said.   The explosion resulted in a small number of casualties,  it said, adding that the airfield was secure and the incident was being investigated. 