Indian security forces raided a Maoist camp in the eastern state of Jharkhand Monday, arresting three rebels and recovering a large cache of weapons.

Local police said the raid, which was part of a three-day “Operation Thunder,” took place near Garhwa, a dense forest area located near the border with Chhattisgarh state.

The weapons cache included can bombs, raw materials used for making explosives, 54 detonators, 17 gas cylinders, three hand grenades, uniforms and Maoist literature.

While Maoist rebels have infiltrated more than 20 Indian states, eastern states are the center of their decades-long insurgency.

Indian leaders have referred to Naxalism as the country's biggest internal security threat. The groups include some of the country's most extreme poor who say they are struggling against state-sanctioned acquisition of their land and natural resources.

The rebels are widely seen as terrorists. Thousands of civilian and police deaths in India are blamed on Naxal attacks since the groups emerged in the late 1960s.