Fifteen bodies were recovered after a fierce gun battle between Sri Lankan forces and an armed group near Batticaloa, police said on Saturday.
Officers acting on a tip-off had launched a raid on Friday night in Ampara Sainthamaruthu and the armed group set off an explosion which triggered the gun battle, the BBC reported.
When troops moved in after dawn on Saturday, they found the bodies of 15 people.
In another raid in the same town, the police found Islamic State (IS) flags, 150 sticks of gelignite, thousands of steel pellets and a drone camera, a military spokesman said.
On Friday, 10 people were arrested from across the country bringing the number detained since the Easter Sunday bombings to 80.
Sunday’s explosions killed 253 people and injured over 500 in one of the bloodiest days in Sri Lanka since the civil was ended a decade ago.