At least 20 Maoists were killed and nine security personnel were injured in a fierce gun battle in a jungle in Naxal-infested Dantewada district of Chhattisgarh, a senior police official said on Tuesday.

Twenty-seven out of the 115-member security personnel team are still missing in the Elampatti-Regadgatta jungles, about 550 km from Raipur, where the gun battle erupted on Monday evening after Central Reserve Police Force and Special Police Force launched an operation to flush out the ultras from there.

"The personnel who have returned from the jungle have told us that at least 20 Naxals were killed in the operation which lasted for about five hours," the official linked with the anti-Naxal operations told PTI.

DIG Vishwaranjan and a top CRPF official said there were no casualties from the security personnel team.

Nine personnel were injured in the encounter, the official with the anti-naxal operation wing said, adding that the condition of three were stated to be serious.

Both security personnel and Naxalites used mortars and light machine guns during the gun battle, he said.

Chattisgarh's Special Task Force and Andhra Police's 'Greyhound' assisted by Union Home Ministry's air wing have launched a search operation in the jungle bordering Andhra, a CRPF spokesman told PTI in New Delhi.

"While 71 personnel came back in the night and 17 returned on Tuesday morning in two batches, 27 are still missing," the official said.

Officials at the police headquarters here said some of the personnel, who had not returned back so far, had received bullet injuries on their legs.

The gun battle prolonged for about five hours but high downpour created a major obstacle for the anti-Naxal operation and during that period the Naxals opened fire from behind, sources said.