Over 15 naxals were killed by security forces in an ongoing encounter near Karregutta hills in Bijapur along Chhattisgarh-Telangana border, ANI reported quoting police officials. Earlier, a woman Maoist was killed during an encounter with security. An anti-Maoist operation, which security sources termed the biggest one launched so far this year, is underway at Chhattisgarh-Telangana border.