At least six persons were killed and three others were injured after an SUV and a tractor-trolley collided head-on on the Tulsipur-Barhni National Highway in Uttar Pradesh's Balrampur on Friday, police said. A 13-year-old boy died in a hospital, police added. The accident took place when the victims were travelling from Laxmanpur to Bhagwanpur, police further said.