Sri Lanka edged closer to all-out war today after a prominent Tamil politician was gunned down on his way to work in Colombo, the capital.

Nadaraja Raviraj, a member of the Tamil National Alliance (TNA) and a human rights lawyer, was shot at close range by unidentified gunman on a motorbike at around 8.30am as he got into his car after giving a TV interview. He died later in hospital. His bodyguard was also killed.

Mr Raviraj, 44, elected twice as mayor of Jaffna in the Tamil heartland, was an outspoken parliamentarian, voicing objections to extra-judicial killings and civilian abductions, and a leading campaigner for Tamil self-rule.

On Thursday, the father-of-two had taken part in a demonstration against the Sri Lankan army's shelling near a school in the north-eastern town of Vaharai which, rebels claimed, killed at least 45 Tamil civilians. The rebels say the military onslaught in the region has displaced 41,000 people.

President Mahinda Rajapakse immediately condemned the shooting, calling it a "cowardly and heinous act" by "those opposed to dissent and political pluralism in a democratic society".

However, Tamil supporters were quick to blame the government. "A democratic voice of Tamils has been silenced," Selvam Adaikalanathan, a fellow party member, told Tamilnet, a pro-Tamil website.

"He had a convincing way of dealing with even the crude bureaucracy of this failed state. He fought from their platform. His voice in the Sri Lankan parliament, and in south, where injustice and oppression originate, was much feared."

Suresh Premachandran, another MP for the TNA, widely considered a proxy vote for the rebel fighters, said: "We understand that a whole magazine has been emptied on them in broad daylight. This is a clear message to Tamil parliamentarians ... don't open your mouth."

Mr Raviraj's murder is the third high-profile assassination in Sri Lanka this year. In July Maha Kanapathipallai, a senior member of a Tamil political party opposed to the Tigers was gunned down in Colombo.

In August Kethesh Loganathan, an ethnic Tamil and deputy head of the government's peace secretariat, was also shot dead in the capital.

Keheliya Rambukwella, a government spokesman, said Sri Lanka had sought the help of London's Scotland Yard to investigate whether the series of killings had been orchestrated by the militants themselves to further destabilise a fraying four-year-old ceasefire agreement.

The motive for the latest killing was unclear and no one has yet claimed responsibility, although it comes amid escalating tension between rebel and government forces.

The Sri Lankan navy today said it destroyed two Tamil Tiger boats laden with explosives in the northeastern port of Trincomalee, killing six rebels. The incident was the latest in a series of tit-for-tat encounters on the seas.

On Thursday, the rebels said they had killed at least 26 Sri Lankan sailors and captured four alive. But the navy said they had only lost seven men and claimed they had sunk 22 Tamil Tiger boats targeting a passenger ship carrying 300 people. With the area off limits to independent observers, there is no way to verify any of the figures.

Surging violence in Sri Lanka has claimed the lives of more than 2,000 civilians, soldiers and rebels this year as analysts fear a return to civil war and the total breakdown of the 2002 Norwegian-brokered ceasefire.

"We have growing concern," Richard Boucher, the US assistant secretary for south and central Asian affairs, said during a visit to India yesterday. "It is very important for both sides to respect the ceasefire. The only way to achieve what they want is through negotiations. They are clearly not going to get a solution through violence."

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