Maoist rebels have killed at least 15 members of India's security forces in an ambush in restive Chhattisgarh state, less than a month before the country is due to hold elections, according to police.

Security officials and police sources said up to 200 rebels attacked a patrol in forests to the south of the state capital, Raipur, before a major gun battle erupted.

Eleven members of the national paramilitary Central Reserve Police Force (CRPF) were killed along with four members of the state police force, said Mukesh Gupta, one of Chhattisgarh's most senior police officers. "As of now, a total of 11 CRPF, four policemen and one civilian have died," Gupta told AFP.

Others suggested the death toll from the biggest rebel attack in almost a year was even higher. Rajinder Kumar Vij, the head of anti-Maoist operations in the central state, put the number of CRPF personnel dead at 15 and said five state policemen had also been killed. There were no figures on the number of Maoist casualties.

Security forces were involved in an operation to clear and open a road in Sukma district when the rebels detonated a landmine and started firing indiscriminately, Vij said. "The attack sparked a gun battle that lasted about three hours."

Several people were injured in the attack and were being airlifted to Raipur for treatment. Security reinforcements have been rushed to the area, along with top state government officials.

Gupta said the attack took place at about 10:30am in a heavily forested area during the operation to clear the road 250 miles (400km) from Raipur.

"The attack was on one of our police parties as the Naxalites (Maoists) have been frustrated at our increased presence in the area," he said. "We don't yet know the casualties on the attackers' side … It is difficult for us because of the topography of the area."

The attack was close to the site of an ambush in May last year on a convoy carrying members of India's ruling Congress party in which 24 people were killed, including the state party president and his son.

The latest deaths will heighten fears of unrest in the Maoists' stronghold in the buildup to the nationwide elections that begin in early April.

Voting will take place in Chhattisgarh in three phases, on 10, 17 and 24 April.

The Maoists, who have been described by the Indian prime minister, Manmohan Singh, as the country's most serious internal security threat, have been fighting since 1967 for a communist society by toppling what they call India's "semi-colonial, semi-feudal" form of rule.

The insurgency is believed to have cost tens of thousands of lives, with violence centred around the insurgent-dominated "Red Corridor" stretching through central and eastern India.

In 2010, rebels killed 35 people in a landmine attack on a bus in Chhattisgarh. The explosion tore the front off the bus, killing 24 civilians and 11 police personnel.