India Maoists kills 15 policemen in Chhattisgarh

India's prime minister has described the Maoists as the country's biggest internal security threat

Maoist rebels have killed at least 15 policemen in the Indian state of Chhattisgarh, police say.

The patrol was on its way to provide security to workers building a road in Sukma district when rebels fired at them, officials say.

Twenty-five policemen were injured in the attack, senior police officer Mukesh Gupta told BBC Hindi.

Chhattisgarh is a stronghold of the rebels who say they are fighting for the rights of the poor.

The Maoists are active in more than a third of India's 600 districts and control large areas of several states in a "red corridor" stretching from north-east to central India.

Tuesday's ambush happened in the Tongpal area of Sukma.

It is not clear whether the rebels suffered any casualties.

Last May, rebels attacked a convoy carrying state Congress leaders and party workers in Sukma, killing 27 people, including some top state politicians.

And in April 2012, rebels kidnapped a senior government official in the district, which is part of the Maoist-dominated Bastar region. Alex Paul Menon was freed after 12 days in captivity.

The Maoist insurgency began in West Bengal state in the late 1960s and has become, according to Prime Minister Manmohan Singh, the country's "greatest internal security challenge".

Major military and police offensives in recent years have pushed the rebels back to their forest strongholds and levels of violence have fallen.

But hit-and-run attacks are still common, killing hundreds of people every year.