Maoist fighters have killed at least 20 members of India's security forces in a massive attack in restive Chhattisgarh state, police say.

Security officials and police sources said on Tuesday the victims were all killed when up to 200 rebels ambushed 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 news agency.

Others suggested the death toll 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 that five state policemen had also been killed.

There were no figures on the number of Maoist casualties.

The 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 told AFP.

"The attack sparked a gun battle that lasted about three hours."

Gupta said the attack took place at around 10:30am in a heavily-forested area during the operation to clear the road some 400 kilometres (250 miles) 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 build-up to the nationwide elections which begin in early April.