A suspected Tamil Tiger suicide bomber killed Sri Lanka's highways minister and at least 12 other people attending a marathon race near the capital Colombo on Sunday.

"Minister Jeyaraj Fernandopulle is dead from the explosion," said Laksman Hulugalla, director general of the media centre for national security.

Television footage showed a ball of fire moving towards the minister as he signalled the start of the run.

K.A Karunarathne, a former top marathon runner, was among the dead. About 100 people, some of them runners in the race, were wounded.

Authorities immediately pointed the finger of blame at the Liberation Tigers of Tamil Eelam (LTTE).

"It's a suicide attack, definitely by the LTTE," said a bomb squad official, speaking at the scene on condition of anonymity.

The official said the explosion took place very close to the minister and involved a large quantity of explosives.

The Sri Lankan military has launched an offensive on the LTTE's northern strongholds in which at least 100 rebel fighters were killed last week, the military said.

The rebels have in the past hit back with bombings in Colombo and in the relatively peaceful south of the island when they have come under military pressure in the north and east.

Fernandopulle, 55, was a member of the government negotiating team for failed peace talks with the Tamil Tiger rebels two years ago.

Sunday's attack took place in the town of Weliveriya, 30 km from Colombo, where Fernandopulle had gone to flag off the marathon.

"I heard an explosion and with that saw a fire ball going towards the minister as he was prepared to signal the race," a witness said.

"After that I had saw people lying in pools of blood."

"COMMITTED DEMOCRAT"

President Mahinda Rajapaksa condemned the killing, blaming it on the LTTE.

"The assassination of such a committed democrat once again shows the total contempt of the LTTE to the democratic process, and its unquestioned commitment to violence and terror to achieve it narrow and limited objectives, that are far removed from the interests of the Tamil people of Sri Lanka," he said in a statement.

"While calling on the people to be calm and collected in the face of such extreme provocation by the forces of terror, I wish to reiterate that this dastardly act will not weaken our resolve to eradicate terrorism from our midst."

The Tigers are fighting for an independent state in the north and east of the island in a 25-year civil war that has killed an estimated 70,000 people.

The LTTE, which usually denies any involvement in such attacks, was not immediately available for comment.

Fernandopulle was the second minister to be killed since January. The minister for nation building, D.M. Dassanayake, died in a roadside blast in the same district, Gampaha.

Foreign Minister Lakshman Kadirgamar was the most senior government leader to be killed in recent years. He was shot dead at his home in Colombo by a suspected Tamil Tiger sniper in August 2005.

In January the government called off a tattered 2002 ceasefire, accusing the rebels of using it to regroup and re-arm, and vowed to fight them militarily.

Analysts say the military has the upper hand in the latest phase of the long-running war given superior air power, strength of numbers and swathes of terrain captured in the island's east. But they still see no clear winner on the horizon.