A claymore mine blast in the northwest killed three soldiers and wounded another two, police said.
The soldiers were killed when an anti-personnel mine exploded in Mannar district 220km (135 miles) north of the capital, Colombo, Brigadier Prasad Samarasinghe said.
Three other soldiers were wounded in the explosion that took place near a public well where the soldiers went for a bath, Samarasinghe said.
Two sailors were killed when a mine exploded as they rode on a motorcycle in northern Jaffna Peninsula, the navy's media unit said.
The blasts blamed on the Liberation Tigers of Tamil Eelam (LTTE) rebels follow Sri Lankan military strikes on Tuesday and Wednesday on rebel-held territory.
Meanwhile, police found five headless corpses near the capital, Colombo, and said they were investigating whether the deaths are linked to the recent upsurge in fighting with Tamil rebels.
The headless male corpses were found in two separate spots at a rubber plantation in Awissawella, a predominantly ethnic Sinhalese area about 35km (22 miles) east of Colombo, said Nevil Wijesinghe, the deputy inspector general of police. Many Tamils work at the plantation.
This week's bloodshed threatens to wreck a 2002 truce that ended two decades of fighting between the government and rebels seeking a separate state in the north of the island.
While both sides in the conflict and the European team overseeing the agreement say the cease-fire still holds, analysts predict that more violence could lead to its collapse.
The Tamil Tiger rebel group says the airstrikes on Tuesday and Wednesday near the northeastern port of Trincomalee killed 12 civilians and forced 40,000 people - mostly ethnic Tamils - to flee their homes.