The attack in the eastern district of Batticaloa on Wednesday came after the government vowed to wipe out the Tigers militarily.
Brigadier Prasad Samarasinghe, a military spokesman, said: "A Claymore (fragmentation mine) attack in front of the Eastern University in Batticaloa has blasted a bus carrying police personnel going on leave.
"Six were killed. There may be more injuries."
Several more were also reported wounded in the attack.
It comes after the military captured a key eastern Tiger stronghold this month that analysts say has emboldened the government to try to end the conflict on the battlefield rather than at the peace talks table.
Wednesday's attack also comes after two naval attacks by suspected Tiger rebels this month, and observers fear the government is underestimating the rebels' military capability.
