The Maoists also operate in parts of Southern India, the State Department said in its latest annual report on terrorism.
It said hundreds of people were killed in conflicts between the government and various leftist extremist groups, such as Naxalites and the Communist Party of India (Maoists), and also in internecine war.
"The Government of India is very concerned over the threat from Leftist extremist groups to internal stability and democratic culture," it said.
The report mentioned that there were at least 971 Naxalite attacks in the first seven months of 2007 which, it said, was approximately equal to that of the entire previous year.
"The Leftist extremists are highly active across a wide swath of India, including the states of Jharkhand, Chhattisgarh, Andhra Pradesh, Bihar, Uttarakhand and West Bengal. They were also active in some areas of Orissa, Maharashtra and Karnataka," the report said.