A Tamil Tiger suicide bomber hit a military checkpoint just outside the Sri Lankan capital on Sunday, killing at least five paramilitary troops and wounding 15, police said.

The bomber stormed into a detachment of the Civil Defence Force near a market in Wattala, a suburb of the capital Colombo, and carried out the attack, a police official at the scene said.

"The suicide bomber managed to get through the first checkpoint and stage the bombing inside the CDF detachment," the officer said.

The wounded were rushed to the main hospital in Colombo as well as a nearby medical facility, doctors said, adding that a total of 15 were taken to hospital.

The explosion came as security in the capital has been stepped up over fears of bomb attacks by Tamil Tiger rebels who are battling a military offensive by government troops in the north of the island.

Separately, the Sri Lankan navy said it had sunk a Tamil Tiger boat carrying an arms shipment together with four guerrillas on board off the island's northern coast on Sunday.

The defence ministry said naval craft detected the boat off the coast of Point Pedro in the north just after midnight and destroyed it, along with the four rebels.

The military has accused the Liberation Tigers of Tamil Eelam (LTTE) of smuggling in weapons from abroad.

"Four LTTE Sea Tiger cadres were also reported to have been killed due to the fierce naval fire directed at the enemy (on Sunday)," the ministry said.

Last week, Sri Lanka's navy said it had destroyed a Tamil Tiger vessel carrying arms and other supplies.

The navy had previously announced that it had sunk virtually the entire fleet of arms-carrying ships belonging to the Tiger guerrillas, who were known to operate a lucrative shipping business through front organisations.

The rebels have been fighting since 1972 for a state for ethnic minority Tamils separate from the majority Sinhalese community.