The de-escalation of the fighting suggests negotiations are ongoing between the armed forces and rebels
The incident occurred on a highway near Barabanki in central Uttar Pradesh at around 12:45 am, Barabanki Superintendent of Police (SP) Yamuna Prasad told Reuters.
"The bus was overloaded and had about 150 passengers in it. It was hit by a truck on the highway," Prasad said.
At least 10 people were injured and sent to the state capital of Lucknow for treatment, he said.
Media reports said a number of passengers were labourers and that the bus had broken down on the highway before the accident. Uttar Pradesh is India's most populous state.
Prime Minister Narendra Modi expressed condolences over the deaths on Twitter.