Police opened fire and killed two suspected Tamil Tiger separatists who lobbed a grenade at a checkpoint in the eastern Trincomalee district early on Sunday, a police official said.
He said another fighter was killed in a separate incident when troops responded after coming under a grenade attack during a routine patrol.
Two soldiers were killed and four others were injured in a landmine attack in the north of the island late on Saturday, a military official said.
The latest violence comes as Norway tries to get the government and Tamil separatists to resume talks on a ceasefire in place since 2002.
The talks, which were to begin on Monday in Switzerland, were put off indefinitely after the separatists accused the government last week of failing to control paramilitary forces, diplomats said.
The ceasefire has also come under renewed pressure in the past two weeks after a spate of bomb attacks.
At least 70 people have been killed in bombings alone while the Tamil separatists say 70 civilians have been slain by pro-government militia or by security forces, a charge the military has denied.
More than 60,000 people have died in three decades of fighting in Sri Lanka.