At least 26 policemen have been killed in a Maoist attack in the central Indian state of Chhattisgarh, police have told the BBC.
Those killed in the latest attack were members of the paramilitary Central Reserve Police Force (CRPF).
Two CRPF personnel were injured and police are searching for casualties.
In late May, more than 145 people were killed when a train crashed in West Bengal after Maoist rebels allegedly sabotaged the rails.
The Maoists, also known as Naxalites, say they are fighting for the rights of rural poor who have been neglected by the government for decades.
Prime Minister Manmohan Singh has described their insurgency as India's biggest internal security challenge.
In the most recent attack, the CRPF members were attacked as they were returning after clearing a road, officials say.
Chhattisgarh anti-Maoist operation head Ram Niwas told the AFP news agency that they were ambushed by a large number of heavily armed militants in Dhodai, 300km (190 miles) south of the state capital, Raipur.
Police said the ensuing gun battle lasted three hours. They say that the injured have been evacuated by helicopter and reinforcements have been sent to the area.
In May a Maoist landmine attack in Chhattisgarh destroyed a bus and killed more than 30 people, most of them civilians.
Maoist supporters saw that armed police were on board the bus and an attack was organised extremely quickly.
Correspondents said that the bus attack showed how powerful the rebels have become in remote regions such as the forests of southern Chhattisgarh. The government said it also demonstrated their barbarity.
Following the attack, the home minister said he would request wider powers to deal with the rebels.
A government offensive against the rebels - widely referred to as Operation Green Hunt - began last October.
It involves 50,000 troops and is taking place across five states - West Bengal, Jharkhand, Bihar, Orissa and Chhattisgarh.
Ministers in Delhi have always accepted that there is a need to tackle the root causes of the rebellion, such as poverty and the absence of effective local government.