Seven policemen were killed in a landmine blast by the suspected ultra-leftist rebels(Maoists) in the eastern state of Chattisgarh, police said on Wednesday.

It was a powerful blast that ripped the roof off a vehicle carrying paramilitary personnel. Five men of CRPF (India's federal police force) were killed on the spot and two died a few hours later, district police chief Ankit Garg said.

The Maoists have called for a 48 hour strike from May 21, in six Indian states demanding the release of three of their leaders.

The landmine attack occurred late on Tuesday in Sukma town in Dantewada district, a hotbed of Maoist activity 515 kilometres (320 miles) south of state capital Raipur.

Prime Minister of India, Manmohan Singh has called it India's biggest internal security threat and has urged state governments to increase welfare measures, especially in Maoist-hit regions, to deal with the insurgencies.

The government had faced criticism for not being able to counter the violence created by the Maoists after 285 policemen were killed in 2010.