All 18 were Commandos of the special anti-Naxal force C 60. Their bodies have been recovered and are on their way to Gadchiroli. Helicopters went there to try and get the bodies out.
The BSF has been deployed in the area.
Naxals have killed two police informers in the past week -- one farm labourer was killed last night, another on October 2.
A Naxal group also exchanged fire with a police commando team east of Laheri village on Thursday afternoon.
The police says that additional forces have been sent to the area, but this is heavily-forested terrain, and will be hard to access.
The attack comes just two days after a Senior Inspector in Jharkhand was found beheaded by Naxals. He had been kidnapped last week.
Spread over 40,000 square kilometers 20 states, in what's come to be known as India's Red Corridor, Naxals are repeatedly described as the biggest internal security threat to the country.
Home Minister P Chidambaram has warned, "As long as the Maoists believe in armed liberation, we have no choice but to ask security forces to them take and engage them."