Maoist rebels have killed 13 police and wounded another 12 in an ambush in a remote part of central India, two senior officers said.
The troops were carrying out an operation deep in a forest when the gunmen attacked, the head of Chhattisgarh state's anti-Maoist operations told the AFP news agency.
"Thirteen security forces were killed in a gun battle with Maoists in a deep forest of Sukma region of Chhattisgarh," said Rajinder Kumar Vij.
Sukma district is 385km south of Raipur, the state capital.
The victims were all with the Central Reserve Police Force (CRPF), a paramilitary unit. Its local head of operations, Zulfikar Hasan, said the attackers had used villagers as human shields.
"The Naxals were using local villagers as shields, hence our force couldn't retaliate with full force as that would also have cost villagers' lives," he told AFP, using a term commonly used for Maoist rebels in India.
The attack is the latest in a simmering conflict that pits the fighters against local and national authorities in the forests and rural areas of mainly central and eastern India.
The armed groups, who say they are fighting for the rights of tribal people and landless farmers, often collect funds through extortion and protection rackets.
India's Home Minister Rajnath Singh on Twitter condemned what he called a "dastardly act of violence".