A suicide blast in Colombo, the Sri Lankan capital, has killed at least six security personnel and injured 15 other people.

The bomber detonated explosives at a checkpoint in the residential Wattala area on Sunday, police said.

"It was a suicide bombing ... We have sent about 15 people to hospital and recovered five bodies," a police officer said.

Security in Colombo has been stepped up recently due to concerns over an attack from the Liberation Tigers of Tamil Eelam (LTTE).

The blast came after the navy claimed that they had killed four LTTE fighters in a boat, about 20km off the Jaffna peninsula, in the northern sea.

Supplies headed for rebel personnel were onboard the 16-metre vessel which exploded and sank, D K P Dassanayke, a navy spokesman said.

The navy suffered no casualties in the attack, Dassanayke said.

The attack came amid intensified attempts by the military to destroy the LTTE's de facto state in the north of the country in recent months.

The government pulled out of a 2002 Norweigan-broakered ceasefire with the LTTE in January.

The LTTE has been fighting since 1972 for a separate homeland for ethnic minority Tamils in Sri Lanka.