Maoists on Friday gunned down at least 11 policemen in an ambush that set off an encounter which left four ultras dead in Chhattisgarh's Bijapur district. A team of Chhattisgarh Armed Police team was ambushed by the Maoists in Metlaperu village forests under Bhadrakali police station area in the district this evening, Additional Director General (Naxal operation) Ramnivas told Press Trust of India over phone. Eleven policemen were killed in the attack and four Maoists perished when the police team retaliated, he said. Two teams of security personnel were sent for patrolling from Bhadrakali and Bhopalpatnam police stations. The Chhattisgarh Armed Police squad came under heavy fire from Naxals in the jungles, the ADG said adding additional security force has been dispatched to the attack site.