They suspected Tamil Tiger rebels had carried out the attack.
There has been a sharp increase in violence in the past few weeks and foreign monitors say a four-year-old ceasefire is under severe strain.
More than 40 soldiers have been killed this month.
Military spokesman Prasad Samarasinghe as said “No-one else is capable of doing this kind of attack in Jaffna except the [Tigers].”
The Tigers have not yet commented on the latest deaths.