BEIRUT (AP) — The extremist Islamic State group shot and beheaded hundreds of tribesmen from eastern Syria over the past two weeks after crushing an uprising they led against the jihadi fighters, activists said Monday.

The killing of members of the Shueitat tribe come as Islamic State group fighters close in on the last government-held army base in the region. Syrian warplanes bombed the extremists’ positions Monday in an attempt to halt their advance.