BAMAKO, Mali — Without warning other governments, Algeria mounted an assault Thursday on the heavily armed fighters holding American and other hostages at a remote Sahara gas field facility, freeing captives and killing kidnappers but also leaving some of the hostages dead and foreign leaders scrambling to find out the fates of citizens trapped inside.

Hours after the raid, there was still no official word on the number of hostages who were freed, killed, or still held captive. Estimates of the foreign casualties ranged from four to 35, though one Algerian official said the high figure was ‘‘exaggerated.’’