A group of an estimated 400 Maoists had ambushed security personnel who were part of a large contingent deployed for a special operation, leading to the killing of at least 22 personnel and injuries to 30 others, official sources said on Sunday. They said the contingent of about 1,500 troops drawn from CRPF S specialised jungle warfare unit CoBRA, some teams from its regular battalions, a unit of its Bastariya battalion, the Chhattisgarh Police affiliated District Reserve Guard (DRG) and others had launched a search and destroy operation along the border of Bijapur-Sukma district around noon after they got inputs of some Maoists presence in the area.