He said an unspecified number of bodies were still trapped in the mangled bus.

Television footage showed bodies laid out on the road next to the wreckage of the bus. The front portion of the vehicle had been almost completely destroyed by the force of the blast.

"The bus was tossed 20 feet [6 metres] into the air and when it fell it created a 10-foot [3-metre] crater," a Chhattisgarh police official said.

Escalating violence

Al Jazeera's Prerna Suri, reporting from New Delhi, said there has been escalating violence in Chhattisgarh in recent months.

"This comes on the heels of the government's policy of the so-called 'Operation Green Hunt' in which they're targeting Maoist rebels.

in depth

  Q&A: The Maoists of India
  Riz Khan: India's Maoist threat
  Inside Story: India's battle against the Maoists
  Timeline: Major Maoist attacks

"So in a sense I think this [attack] is retribution and a message to the central government that this is clearly not working."

Eight paramilitary soldiers were killed when Maoists blew up an army vehicle in Chhattisgarh's Bijapur district.

On April 6,  75 security personnel were killed after hundreds of Maoists ambushed and attacked policein Dantewada.

The government offensive against the Maoists began late last year in the forests of the so-called "Red Corridor" that stretches across north and eastern India.

The operation involves 56,000 paramilitary forces in six states in addition to local police.

Palaniappan Chidambaram, the home minister, said the recent attacks may trigger a change in the government's strategy against the Maoists in some states.

"We have suffered some major setbacks. We need to revisit tactical operations in Chhattisgarh and Orissa [states]. These are the flashpoints," he told NDTV news channel.

The chief ministers of several states racked by Maoist violence had asked for military support, he said.

Thousands of people have been killed in the Maoist's four-decades-old insurgency which the Indian government has described as the country's gravest internal security threat.

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.

Chidambaram has earlier said the government needs to tackle the root causes of the insurgency and has offered talks with the fighters, on condition they renounced violence beforehand.

Senior Maoist figures have said they will talk only if the government puts an end to the national offensive against them.