Police said the impact of the explosion was so strong that sounds of it could be heard some kilometres away adding the victims were at the Viewland area at the time of the incident.
It was not immediately known whether the bomb was planted by suspected militants or hurled at the spot, police said.
Ukhrul district headquarters was in the limelight following the killing of two persons in alleged police firing on rallyists for defying prohibitory orders under section 144 CrPC about two weeks ago.