Rasiah Ilanthirayan, an LTTE spokesman, denied the Tamil Tigers were responsible for the bomb.
"I only have to say that we have nothing to do with it," he said.
Four special task force police commandos were among the wounded in the blast during Monday's rush-hour, Samarasinghe said.
|"It looks like a claymore mine was placed on the roof of a roadside shop and detonated with a remote controlled device as the truck passed" |
Nine commandos were in the truck that was heading towards Colombo from a training facility at Katukurunda.
Hundreds of troops pass through the suburban area just 15km south of the centre of Colombo every day.
"It looks like a claymore mine was placed on the roof of a roadside shop and detonated with a remote controlled device as the truck passed," a police officer at the scene said.
The owner of the shop was telling police that the premises had been broken into when the blast ripped through, he added.
The bombing came just days after an army bus was targeted near the entrance to Colombo port, killing one soldier and injuring six other people, and a fierce naval battle off the Jaffna peninsula on Thursday.
More than 5,000 people have been killed in a resurgence of the conflict since December 2005 despite a ceasefire being agreed in February 2002.
Separatist violence in Sri Lanka has claimed more than 60,000 lives so far.