A bomb blast on a crowded bus near Colombo, the capital of Sri Lanka, has killed 21 and injured more than 50 people in what is thought to be the latest in a string of recent attacks by the separatist Tamil Tiger movement.

"Twenty-one people have been killed, eight of them are women," a police spokesman said. The attack was the third targeting civilians in and around Colombo in less than two weeks.

Violence between the army and the separatist Liberation Tigers of Tamil Eelam (LTTE), which is listed as a terrorist organisation by the United States and the European Union, has intensified since the government formally pulled out of a six-year-old ceasefire pact in January.

Colombo has since launched an offensive in the Tigers' heartland, in the north of the country, vowing to crush them by the end of the year.

However, most analysts suggest a quick victory against a force that has been fighting for a separate Tamil homeland for 25 years is unlikely and that recent bomb attacks in the relatively peaceful south of Sri Lanka have proved that the group can strike deep within Government-held territory.

Survivors said that the latest blast, thought to have been caused by a claymore mine, was powerful enough to topple the packed bus onto its side.

Shanika Priyadharshani, a 21-year-old office worker who had been travelling on the bus, told AFP: "I blacked out for a while. There was black smoke, people were dead around me. I shouted for help and someone pulled me out."

An army spokesman said the LTTE was responsible. He said it was a "cowardly attack by a ruthless organisation." The attack is the deadliest in the country since April 25, when at least 24 people were killed in a similar bus bombing. The most recent fatal bomb attack was on May 26, when at least nine people were killed on a train and more than 80 injured.

On Wednesday, 18 people were hurt when suspected Tiger rebels set off a bomb alongside a busy commuter train.

Each of the recent attacks has followed LTTE complaints that government forces have killed civilians in roadside bombings in the rebel-held north. It claims that eight civilians have been killed in such government attacks in the past week.

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