Sri Lankan President Mahinda Rajapaksa has vowed "not to stop until terrorism has been defeated".

The blast hit the Colombo-Panadura train in Dehiwala station, in a suburb of the capital, Colombo.

The authorities have blamed Tamil Tiger rebels for the attack, the latest in a string targeting public transport.

The Tamil Tigers routinely deny involvement in bombings in Sri Lanka.

'Defeats and setbacks'

"My government and I are committed to defeating terrorism. No-one should have expectations that there will be a let-up in the battle against terrorism because of the frenzied attacks by the Tamil Tigers," the president said.

"I will leave no room for terrorism in this country."

He accused the rebels of "desperately trying to whip up communal strife" in the country and pledged that the government would not allow this to happen.

Mr Rajapaksa said the bomb blast on the train at Dehiwala followed the discovery of three other bombs placed in passenger buses, all of which were found by bus employees and passengers.

It was clear, he said, that in the face of defeat the rebels were increasingly attacking innocent civilians.

He also stressed that the government's battle against terrorism was not directed against the Tamil people and was being conducted to protect their rights.

The police say that 12 suspects have been arrested in connection with Monday's bombing.

The blast came four days after the rebels blamed government forces for an explosion which killed a group of people in Tiger-controlled territory.

Military offensive

Meanwhile, battles have been continuing in the north of Sri Lanka, where the military is carrying out an offensive aimed at crushing the rebels by the end of this year.

Air force helicopters attacked Tiger positions on Monday, a day after fighting on the ground killed 21 rebels and one soldier, according to the military.

Correspondents say that the number of attacks in the south has increased as the fighting has intensified in the north.

The Tigers have fought for a generation for an independent state for the Tamil minority in the island's north and east.

About 70,000 people have been killed since the civil war began in 1983.

Violence in the island nation increased after the government formally pulled out of a ceasefire agreement in January.

(Courtesy : BBC )