Tamil Tiger rebels bombed an army bus in the Sri Lankan capital on Wednesday killing four people and wounding 21, military and hospital officials said, the latest in a series of attacks as renewed civil war deepens.

The rebels set off a roadside bomb outside a modest hotel in area of Colombo where the army and air force headquarters are both located, the military said. Two of the dead were soldiers and two were civilians.

"There was a Claymore (mine) attack targeting an army bus carrying troops," a military official at the scene told Reuters, asking not to be named in line with policy.

"It's definitely an LTTE Claymore," he added, referring to the type of explosive device military analysts say are a hallmark of attacks by the Liberation Tigers of Tamil Eelam (LTTE).

He said there were "about 11 soldiers" on board the bus at the time. Police cordoned off the area.

The Tigers, who are seeking to carve out an independent state in north and east Sri Lanka, denied involvement in the attack with a routine disclaimer.

"We have nothing to do with that," rebel military spokesman Rasiah Ilanthiraiyan said by telephone from the Tigers' northern stronghold of Kilinochchi. "It is up to the government to find out who did it."    

Ilanthiraiyan said the Tigers had noted a military build up on the government side of heavily-defended forward defence lines that separate state from rebel-held territory in the north.

"What can I say? We can see more clashes in the future," he said.

The blast is the latest in a series of attacks on military installations and buses carrying troops blamed on the rebels in recent months, and comes as the state and Tigers fight near daily land and sea clashes mainly in the north of the island.

The attack came a day after a prominent opposition Tamil parliamentarian was shot dead at a Hindu temple in the capital, a killing the opposition blamed on the government for failing to ensure his security.

Well over 5,000 people have been killed in fighting between the foes since early 2006, taking the death toll since the war erupted in 1983 to around 70,000. While the military has captured vast swathes of territory from the rebel in the island's east, analysts say there is no clear winner on the horizon and fear the war could grind on for years. - Reuters