Police said three migrant workers from Sri Lanka's majority Sinhalese community were shot in an open field in the town of Vavuniya on Thursday.
The town borders territory held in the north of the island by the Liberation Tigers of Tamil Eelam (LTTE).
A police official said: "We initially suspected the victims were local Tamils, but other labourers have now identified them as those who came here looking for work."
The defence ministry blamed the LTTE for the workers' killings.
The ministry also said Tamil Tigers shot and killed a civilian in nearby Trincomalee district, adding that commandos had repulsed an attack there and killed two rebels.
On Wednesday, two bomb blasts in Vavuniya wounded 18 people, including two soldiers.
Meanwhile, three soldiers died in two skirmishes late on Wednesday on the Jaffna peninsula, and a fourth was killed in a mortar bomb attack in Trincomalee district, the ministry said.
The Sri Lankan government denied on Wednesday that it had agreed to hold unconditional peace talks with the LTTE, as announced by international mediators on Tuesday.
"The Government of Sri Lanka is highly disturbed with regard to the statement made by the Norwegian facilitator, as the government neither agreed for unconditional talks nor was it consulted," the government said in a statement.