The blast took place in Ukhrul district, when a BRTF truck was carrying an unknown number of people, including the nine injured.
"Eight BRTF workers and the six-year-old daughter of the BRTF driver were injured when a powerful bomb planted on the road exploded," Manipur police chief M.K.Das told IANS.
Of the nine injured, the condition of three people, including the girl and her father, the truck driver, is critical.
The three have been rushed to the Regional Institute of Medical Science in state capital Imphal.
Das said preliminary reports suggest that the blast could have been triggered by a remote-controlled device.
"A forensic team has started collecting evidence from the site of the blast. We would be able to know only tomorrow (Tuesday) which rebel group is responsible for the blast," he said.
No arrests have been made so far, he said. No rebel group claimed responsibility for the blast.
There are more than 20 rebel groups active in Manipur, a state bordering Myanmar. Their demands range from secession to greater autonomy.