Indian security forces have killed at least 16 extremist leftwing rebels in an overnight encounter in the central state of Chhattisgarh, a police officer said Friday.

"Acting on a tip-off, a team from the Central Reserve Police Force (CRPF) surrounded an area in Basugada in the state's Bijapur district where the Naxalites holed up. Seeing the security forces, the rebels opened fire forcing the CRPF personnel to retaliate. While 16 Naxalites were killed, some six security personnel were injured in the encounter that went on for hours," he said.

The injured personnel have been rushed to a hospital where the condition of some are said to be serious.

"We have arrested two Naxalites and seized a huge cache of arms and ammunition from them," the official added.

The leftwing rebels are active in nearly half of India's 600 odd districts.

Indian Prime Minister Manmohan Singh had described the Naxalites as the country's biggest internal security threat.