At least four Indian security forces personnel have been killed and three others injured in an attack by suspected left-wing Naxalite rebels in the eastern state of Odisha on Tuesday, a police official said. \"A group of suspected armed Naxalites triggered a landmine explosion on a convoy of the security forces in the state\'s Koraput district. While four were killed on the spot, three others have been rushed to a local hospital where their conditions are said to be serious,\" he said on condition of anonymity. The security forces have launched a massive manhunt in the area to nab the culprits, the official added. Naxalites are active in at least seven Indian states, and they often unleash attacks on Indian security forces, in which many personnel have been killed in the last five years. Indian Prime Minister Manmohan Singh has described Naxalite insurgency as the country\'s biggest internal security threat.