NEW DELHI In the past five years, Maoist rebels have emerged as the most potent threat to India's internal stability and left the authorities groping for a response to their increasingly audacious attacks.

Authorities blamed the Maoists for a railway attack in the eastern state of West Bengal early on Friday that derailed an express train, killing at least 65 people.

The origins of the left-wing insurgency can be traced back to a 1967 peasant uprising in the remote village of Naxalbari in West Bengal. In India, the rebels are most commonly referred to as “Naxalites”.

The uprising was eventually crushed by police but over the years the Maoists expanded their base, enlisting thousands of villagers and landless tribals who now form the movement's core.

The insurgency has spread to 20 of India's 29 states with the main centre of activity in the so-called “Red Corridor” covering the natural resource-rich states of Jharkand, West Bengal, Orissa, Bihar, Chhattisgarh and Andhra Pradesh.

Tribal groups and many rural areas have been left behind by India's economic development, and poverty and discontent with local government corruption are seen as fuelling support for the insurgents.

Their current strength is estimated at between 10,000-20,000 guerrillas, who operate out of jungle camps where they undergo weapons and ideological training.

Between 2005 and 2010, the Maoists killed more than 1,220 security personnel and about 2,640 civilians, according to Home Ministry data.

Using their abundant supplies of automatic weapons, landmines and improvised explosive devices, they usually target police patrols, alleged informers, rail tracks, schools and government buildings.

Security forces say their principal sources of funding are from abductions, extortion and looting. They have also set up unofficial administrations in some rural areas to collect taxes.

As the years have passed, so the rebels have grown more brazen in their operations.

In 2007, they assassinated a federal MP and engineered a mass prison break for 300 of their jailed fighters. The next year witnessed the sinking of a boat carrying elite commandos, while in April 2009 they briefly held an entire train with 300 passengers hostage.

“The Maoists have three things on their side — stealth, speed and surprise. In that sense they have the initiative,” said analyst P.V. Ramana from the government-funded Institute of Defence Studies and Analyses (IDSA).

The Indian government launched a centrally coordinated anti-Maoist offensive in November 2009, dubbed Operation Greenhunt.

The surge saw over 60,000 paramilitary police and state police pushed into the worst-affected states along with extra funds for modernising the forces and development packages.

But the operation produced little in the way of tangible results as the Maoists responded with a series of deadly attacks that raised questions over the government's reluctance to deploying the military against the rebels.

The pressure to up the stakes increased after 76 policemen were massacred in the worst single attack to date in April, and a landmine blast that killed 24 civilians and 11 police in May triggered a government review of its strategy.

Home Minister P. Chidambaram acknowledged that changes were needed and said he would request wider powers.

The minister noted that the chief ministers of four of the worst-hit states had asked for air power to be used against the rebels — a measure the government has so far refused to sanction.

Indian forces killed 900 Maoists between March 2005 and February 2010 and over 200 are in Indian prisons, according to a report by the IDSA.—AFP