Maoist rebels have killed 12 people in two poll-related blasts in the instability-hit region of central India, police said, highlighting security threats aiming to disrupt a marathon election in the world's biggest democracy.

Saturday's attacks came as Indians cast ballots in the resort state of Goa and in the far-flung northeast in another round of the multi-phase polls that wind up on May 12 with results due on May 16.

Six polling team officials were killed when Maoists blew up their bus in the state of Chhattisgarh, senior police officer Gurjinder Pal Singh told the AFP news agency.

"The Maoists triggered the landmine blasts," Singh, a key official in ensuring election security in the state, told AFP.

Five security men engaged in an election safety operation and another victim were killed in a separate landmine blast that created a huge crater in the road.

The men were blown up near Darbha, where Maoists massacred the top leadership of the Congress party of Chhattisgarh last May.

The blasts, just an hour apart, came just days after Maoist rebels killed three soldiers guarding polling officials in Chhattisgarh, underscoring the security challenges facing election organisers in India.

Separatist and Maoist insurgencies afflict large swathes of India's northeast, northwest and central regions.

Vowing to prevent the rebels from disrupting the vote, the government has deployed tens of thousands of police and paramilitary soldiers to guard polling booths in insurgency-wracked areas. But the rebels have only stepped up their attacks while also asking citizens to boycott the vote.

"Internal security threat"

The electoral race over the 543-seat lower house of parliament kicked off earlier in the week. The opposition Hindu nationalist Bharatiya Janata Party are tipped to win, ousting the ruling Congress party after a decade.

The vote is held in stages to allow security forces to be moved around the country to protect voters.

Maoist rebels, who say they are inspired by Chinese revolutionary leader Mao Zedong, killed 16 people in a massive attack on security forces in central India in March in the deadliest attack so far this year, heightening fears of more unrest in their stronghold.

The Maoists, who have been described by Prime Minister Manmohan Singh as the country's most serious internal security threat, have been fighting since 1967 for a communist society by toppling what they call India's "semi-colonial, semi-feudal" form of rule. They seek greater share of wealth from the area's natural resources and more jobs for the poor.

The violence is believed to have cost tens of thousands of lives, with much action focused around the restive-dominated, so-called "Red Corridor" stretching through central and eastern India.

The rebels are believed to be present in at least 20 states but are most active in Chhattisgarh, Odisha, Bihar, Jharkhand and Maharashtra, occupying thousands of square kilometres (miles) of land.