Five civilians were killed and over 30 others were injured when a bomb exploded inside a passenger bus traveling near the Sri Lankan capital of Colombo late Friday afternoon, defense officials said.

"The explosion happened around 6:40 PM local time (1310 GMT) and the bus was full of passengers at that time," said the military spokesman Brigadier Prasad Samarasinghe.

The privately owned bus was traveling between Nittambuwa, about 39 km northeast of Colombo and Giriulla when it exploded on the main Colombo-Kandy A1 highway, according to police.

An officer from the Media Center for National Security said the military suspected the bomb attack was the work of the Liberation Tigers of Tamil Eelam (LTTE) rebels.

Samarasinghe said it was too early for the investigators to state what caused the blast -- if it was a hand grenade or a powerful bomb.

The bus was immediately in flames after the explosion, eye witnesses said.

The injured were admitted to the nearby Wathupitiwala hospital. Officials of the hospital said the condition of 10 civilians was critical.

The authorities in Colombo's main hospitals said they were on alert for casualties as around 80 passengers were thought to be traveling in the ill-fated bus at the time of the blast.

Keheliya Rambukwela, defense spokesman of the government said earlier that the LTTE is carrying out terror attacks against civilians unable to bear the continuous defeats at the hands of the security forces.

The Norwegian backed peace process and the truce agreement signed between the government and the LTTE in 2002 are currently stalled in the face of escalating violence since December 2005.

More than 3,800 people were killed despite pleas from the international community on both sides to revive the process of direct negotiations.

(Xinhua News Agency January 6, 2007)