BHUBANESWAR, India - Maoist rebels killed four paramilitary troops and injured two others by setting off a landmine on a highway in eastern India on Tuesday, police officials said.

The powerful blast sent the soldiers’ vehicle flying through the air, with photographs showing the upturned, mangled van lying in a crater in Orissa state’s Koraput district, located 366 kilometres (228 miles) from the capital Bhubaneswar.

“The blast killed Four Border Security Force personnel and grievously injured two others”, Prakash Mishra, the state’s director general of police, told AFP.

The incident happened Tuesday morning, while the troops were driving to a railway station to catch trains taking them home for vacation, a police source told AFP.

Orissa is part of a cluster of impoverished, densely forested states in eastern and central India that are home to a Maoist revolutionary movement, described by the government as the country’s most serious internal security threat.

A group of more than 50 Maoist guerrillas ambushed a police patrol in the bordering state of Jharkhand last month, killing the district police chief and three other officers.

In May, the rebels killed 24 people including local leaders of the country’s ruling Congress party during an ambush in the neighbouring state of Chhattisgarh. The revolutionaries have waged a decades-long battle across central and eastern India to overthrow local and national authorities. They say they are fighting for the rights of the poor and landless farmers in India.

The insurgency began in 1967 as a network of leftwing ideologues and young recruits in the village of Naxalbari outside Kolkata, the capital of West Bengal state. The Naxalites are now estimated to have 30,000 fighters and have pledged to violently overthrow the Indian government.

They control vast swaths of the so-called Red Belt in central and eastern India, where troops and officials rarely venture. The rebels are thought to operate in 20 of India’s 28 states. Prime Minister Manmohan Singh says the rebels are India’s biggest internal threat.