"The rebels were in an advantageous position and fired at the convoy indiscriminately for about 15 to 20 minutes, leaving no scope for retaliation."
Some 19 armed groups are active in Manipur, which borders Myanmar, and their demands range from secession to autonomy.
"This is just one of a number of separatist struggles being carried out by smallish groups of people in several different states in India's remote northeast," Laurence Lee, Al Jazeera's correspondent, said.
"These areas are so far from India's seat of power in Delhi ... that their argument is that they are simply too far away for Delhi to be able to give them any real quality of life."
The attack came a day after the state voted in the last stage of three-part assembly polls which were spread over two weeks.
Army and paramilitary troopers have launched a massive search operation to track down the people behind the attack.
"The terrain is hilly and probably the militants have by now moved out of the area to their bases," another senior police official said.
No group has claimed responsibility for the ambush, the biggest such attack in Manipur in five years.