At least 13 security personnel were killed on Friday when Naxals triggered a blast and then opened fire on a patrol party near Tongada village, bordering Andhra Pradesh, in the Maoist-affected district of Bijapur in Chhattisgarh.

The patrol party, comprising 21 police personnel, was returning from Terla village when the rebels unleashed the attack.

Inspector-General of Police (Anti-Naxal Operations) Girdhari Nayak told UNI over telephone that initially 18 personnel went missing after the ambush. However, five returned.

Meanwhile, IG (Bastar Range) R K Vij said additional forces were rushed to the spot for rescue and to undertake combing operations.

"A clear picture about casualties will emerge only after the security forces take control of the area," he added.

Bijapur is one of the districts of Bastar region where Communist Party of India (Maoist) cadres are active. The ultras stepped up violence since June 2005 after local tribals launched 'Salwa Judum,' a movement to isolate insurgents and generate awareness among the masses against Naxal violence.

According to rough estimates, more than 700 people -- including scores of personnel -- lost their lives over the past two years after rebels stepped up violence. More than 50,000 people deserted their villages and sought refuge in camps set up by the state government in Dantewada district.