A bomb ripped through a crowded passenger bus near Sri Lanka's capital during the morning rush hour Friday, killing at least 21 people and wounding an additional 47, officials said.

Military spokesman Brig. Udaya Nanayakkara blamed the attack in a Colombo suburb on Tamil Tiger rebels. Rebel spokesman Rasiah Ilanthirayan did not respond to calls seeking comment but the rebels routinely deny such attacks.

If carried out by the rebels, the attack would show their ability to strike deep inside government territory despite a maze of security checkpoints around the capital and its suburbs.

Authorities have asked the public to remain vigilant in the wake of several bombings blamed on the rebels, including an attack at a Colombo bus station that killed 26 people last month.

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.

The Tigers have fought since 1983 to create an independent homeland for ethnic minority Tamils, who have been marginalized by successive governments controlled by the majority Sinhalese. More than 70,000 people have been killed.