At least 257 people were killed in a military plane crash near an airfield outside the Algerian capital on Wednesday, the media reported.An Ilyushin I76 troop carrier belonging to the Algerian Air Force crashed in the military airbase of Boufarik in Blida province, Xinhua news agency quoted the Defence Ministry as saying.Ennahar private television channel said the plane carried over 250 troops and was departing for the southwest province of Bechar, before crashing in an unpopulated green field near the airport.Footage captured by witnesses showed a huge plume of black smoke billowing from near the runway.Army Chief of Staff Lt Gen Ahmed Gaid Salah extended condolences to the families of the victims but did not provide a death toll. The cause of the crash was unclear and an investigation was launched, the Defence Ministry said.Authorities said that 300 fire-fighters equipped with 40 ambulances and 40 fire trucks were at the crash site.