A navy official said on condition of anonymity on Thursday that the blast occurred in Chettiukulam in Vavuniya district, 210km north of the capital, Colombo.
There were about 20 sailors in the convoy moving towards their base in the northwestern town of Mannar. Another eight sailors were injured in the blast, he said.
The latest incident brings to 69 the number of Sri Lankan security forces killed in attacks blamed on the Tamil Tigers, endangering a nearly four-year-old ceasefire that halted nearly 20 years of civil war.
Call for talks
The Liberation Tigers of Tamil Eelam rebels have denied involvement in any of the attacks but the style of planning and execution of attacks make them the main suspects.
The attack came as the European Union called on the government and the LTTE to hold talks, warning them that a return to civil war would cause "massive human suffering."
The government and the rebels have said they are ready for talks, but issues such as a venue remain in dispute.
Eric Solheim, the Norwegian who brokered the 2002 ceasefire, is scheduled to arrive in Sri Lanka on 23 January to push for fresh talks.
The rebels want to establish a separate Tamil homeland in the island's northeast, alleging discrimination by the majority Sinhalese-dominated state.
The civil war has claimed 65,000 lives and displaced 1.6 million since 1983.