Tamil Tigers blamed after bus bombing kills 21 in Sri Lanka

It is the second attack in three days targeting civilians in and around Colombo. Tamil Tiger rebels were blamed.

If the group was behind the attack, it shows their ability to strike deep inside government territory despite a maze of security checkpoints around the capital and its suburbs.

The roadside bomb was detonated at about 7:35am in the Colombo suburb of Moratuwa as the bus went by.

Bombing: Special Task Force soldiers stands alert at the scene of a bus that was the target of a roadside bomb attack in a Katubdda

A curfew has been imposed in the area to give soldiers and police a chance to search for suspected rebels.

The explosion shattered the bus' windows and peppered it with shrapnel. A 45-year old man who identified himself only as Nalaka said he was thrown from his motorcycle by the explosion.

'When I got up I saw the bus and quickly got into it. Some people lay dead. Some others were bleeding,' he explained.

'I heard somebody screaming 'help, help,' and I rushed to him, but I could not move him because he was heavy.'

The rebels, blamed for scores of suicide bombings and other attacks on civilians, are listed as a terrorist group by the United States, the European Union and India.

Alert: Sri Lankan security forces personnel stand guard by the bus on the outskirts of Colombo

Rescued: Medical staff push a cart carrying an injured survivor of the bus explosion

They are believed to be behind a blast on Wednesday that targeted a passenger train in Colombo and wounded 19 people.

The Tigers have fought since 1983 to create an independent homeland for Sri Lanka's Tamil minority, which has been marginalized by successive governments controlled by the Sinhalese majority. More than 70,000 people have been killed.

Since a long-standing peace process broke down nearly two years ago fighting between the Tigers and government forces has steadily has escalated along the northern front lines and in the east, where the military last year overran rebel-controlled enclaves.

Investigation: A Sri Lankan policeman is silhouetted against the shattered windshield of the ill fated passenger bus

But the rebels still run a de facto state in much of northern Sri Lanka, and the government says it plans to completely crush the insurgents by the end of the year - a threat diplomats and other observers doubt Colombo has the ability to carry out.

So the fighting continues and the death toll rises, with civilians bearing the brunt of the violence.

Since the start of the year, more than 200 civilians have been killed by bombings in rebel and government territory, according to the U.N. Office for the Coordination of Humanitarian Affairs.

A blast blamed on rebels on a passenger train last month killed eight people and wounded 70 others near Colombo. Also last month, a bomb explosion deep in rebel-held territory killed 16 people.