About 20 of the victims were injured critically, he said.
The blast was under investigation, but officials suspected a leading separatist group, the United Liberation Front of Asom, may have been involved because it has been active in the area, Singh said.
ULFA is one of several rebel groups that are fighting for autonomy or independence in India's northeast.
The militants say India's national government exploits the region's rich natural resources but does little for the area's indigenous people, most of whom are ethnically closer to people in Myanmar and China than the rest of India.
Last week, several ULFA commanders announced a cease-fire and called for an end to their 30-year rebellion.
Other factions of the militant group have rejected the truce, and analysts expected violence to flare as the rebel leaders struggled for control.