Security authorities are blaming Maoist rebels for a bomb that exploded underneath a police bus in central India, killing at least 12 police officers and injuring 28 others. The blast from an improvised explosive device planted on a country road around 20 miles from Gadchiroli in eastern Maharashtra state destroyed the bus, which was part of a police convoy. The vehicle ended up on its side next to a 3-foot-deep crater in the roadway. All the dead and wounded were from a unit of the Central Reserve Police Force, a Times of India report said. Police engaged in a brief gun battle but no arrests, deaths and injuries were reported from the fighting, the Times said. The CRPF, a national backup paramilitary force, is often the target of rebel groups. The paramilitaries are deployed to areas where local police and military authorities need assistance in maintaining peace, especially where they are combating rebel groups. The area were the bomb exploded is in what authorities consider to be one of the state\'s most sensitive regions and where police and army units are battling Maoist rebels, the Times of India said. Maoists are often called Naxalites after the village of Naxalbari in West Bengal state where the group started in the late 1960s. Naxalites -- members or former members of various legal communist splinter groups -- are demanding more of the wealth from the region\'s natural resources, especially from new mining projects, be spread among the mainly rural poor. The eastern part of Maharashtra state is close to what the federal government calls the Red Corridor -- the mineral-rich but remote and poor eastern states of West Bengal, Jharkhand, Bihar, Orissa, Chhattisgarh and northern parts or Andhra Pradesh. Around 600 people are killed each year in clashes between Naxalites and security forces. But a surge in deaths to more than 1,100 in 2009 prompted the government to launch Operation Green Hunt, an ongoing military offensive by 50,000 CRPF soldiers backing tens of thousands of regular policemen. The latest attack follows Naxal threats that it would repeat the massacre of Laheri where 17 police personnel were killed in October 2009, the Times said. In January, police in India\'s northeastern state of Manipur blamed rebels for violence during voting in which seven people were killed including a young girl. A gun battle between CRPF troops and two rebels in the polling station left one of the suspected gunmen and six people dead, the Times of India reported at the time. The dead were one police officer, four polling officials, a civilian and a 14-year-old girl who had accompanied her parents to the polling station. The dead gunman was believed to be a Naga rebel whose organization\'s fight is for more autonomy for the state of Nagaland which lies directly to the north of Manipur. Nagaland is an isolated and economically poor state of around 8,600 square miles. About half the population of 3 million lives in villages.