RAIPUR, India (AFP) -- Maoist rebels ambushed and killed 75 policemen in the jungles of central India yesterday in the worst ever massacre of security forces by the left-wing extremists, officials said.

A patrol from the Central Reserve Police Force (CRPF) was attacked at dawn in the state of Chhattisgarh and when reinforcements rushed to the scene they were surrounded by hundreds of heavily armed rebels.

In a hail of automatic gunfire and landmine explosions, 75 officers were killed, seven were injured and a heavily armoured anti-mine vehicle sent to retrieve the wounded was blown up, government officials and police told AFP.

Home Minister P Chidambaram expressed shock at the *lo**bath in the Maoist stronghold of Dantewada district and said the men had "walked into a trap".

"This shows the savage nature of the Maoists -- the brutality and savagery which they are capable of," Chidambaram told reporters in New Delhi.

Home Secretary Gopal Pillai told AFP the death toll was 75 and seven injured. A spokesman for the CRPF said this included 74 paramilitary forces and a local policeman.

The guerrillas have stepped up attacks in response to a government offensive against them that began late last year in the forests of the so-called "Red Corridor" that stretches across north and eastern India.

The insurgency began in the state of West Bengal in 1967 in the name of defending the rights of tribal groups, but attacks have since spread to 20 of India's 28 states.

Prime Minister Manmohan Singh has labelled the insurgency the biggest internal security threat to India.

Tribal groups and many rural areas have been left behind by the country's economic development, and the poverty and discontent with local government corruption is seen as a major source of Maoist support.

Chidambaram has previously said the government needs to tackle the root causes of the insurgency and he has offered talks with the rebels -- on condition they renounced violence beforehand.

Senior Maoist figures have said they will only talk if the government puts an end to the national offensive, codenamed Operation Green Hunt, that involves 56,000 paramilitary forces in six states in addition to local police.

Prime Minister Singh expressed "shock and grief over the horrific incident", his spokesman said.

French Foreign Minister Bernard Kouchner offered the Indian government and its people "France's unreserved support and determination to remain at their sides in the battle against terrorism".

So far, New Delhi has resisted using the military against the insurgents, though the deaths yesterday prompted calls for a larger assault.