The blast targeted a vehicle carrying members of the Central Reserve Police Force in the Dantewada district of Chhattisgarh state, reports said.
Officials say the death toll is likely to increase.
The Maoists say they are fighting for communist rule and greater rights for tribal people and the rural poor.
Their insurgency began in the eastern state of West Bengal in the late 1960s, spreading to more than one-third of India's 600-plus administrative districts.
Chhattisgarh is often hit by Maoist violence.
Former Indian prime minister Manmohan Singh described India's Maoist insurgency as its "greatest internal security challenge".
Major military and police offensives in recent years have pushed the rebels back to their forest strongholds and levels of violence have fallen.
But hit-and-run attacks are still common, killing hundreds of people every year.
At least seven paramilitary policemen have been killed in a landmine blast allegedly triggered by Maoist rebels in central India, officials say.