Three blasts went off in Dimapur, the commercial hub of Nagaland state, on Saturday and killed 21 people, police said.
The attacks marked the deadliest violence in the state since New Delhi struck a truce with Nagaland’s main separatist group, the National Socialist Council of Nagaland, in 1997.
The blasts went off at a railway station and in a busy shopping area.
The railway platform was packed with passengers waiting to board a train.
“Up to 85 people were wounded in the three blasts and the number of casualties could go up considering the serious injuries received by many of the people,” the police spokesman said.
“We are not sure who could be behind the blasts,” he said.
The attacks in Christian-majority Nagaland could have been set off by several smaller separatist groups that are not part of a truce with the Indian government, the police officer added.
Another bomb exploded in the neighbouring state of Assam killing one man and injuring seven others.
Police said the two attacks on Saturday appeared to be unrelated.
The blast in Assam was likely to be the work of Bodo tribals who are fighting for a separate state, police said. The bomb went off in a market in Kokrajhar district, 150km west of the state’s main city of Guwahati.
India’s north east is home to dozens of underground groups, some fighting for greater autonomy, or statehood and others for secession. The groups accuse the federal government of plundering the region’s rich resources and neglecting the local economy.