Seven Naxals were killed and a policeman sustained serious injuries in an encounter in Chhattisgarh's Dantewada district on Tuesday.Security forces gunned down seven Naxals in the jungles of Tetemdagu village, nearly 18 kms from Kistaram town, in the district, Dantewada Superintendent of Police Amresh Kumar Mishra said, adding that a policeman was also seriously injured in the ambush.