Gunmen assassinated a pro-rebel Tamil MP in the capital Colombo on Friday as Sri Lanka's warring parties fought fresh battles and faced rising international condemnation over civilian killings.
Unidentified gunmen shot dead legislator Nadarajah Raviraj, 44, and his bodyguard as they drove to work, police said. A few minutes earlier, Raviraj had appeared on television accusing security forces of killing civilians.
President Mahinda Rajapakse said the slain politician was a personal friend and expressed shock at the news of his assassination, his spokesman Chandrapala Liyanage said.
"The President ordered the inspector-general to make sure that they get to the bottom of this political assassination," Liyanage said. "He was shocked and saddened to hear about it."
He said the President had also asked the foreign ministry to request Britain's help with the investigation.
Raviraj is the second legislator from the Tamil National Alliance to be killed in the past year. MP Joseph Pararajasingham was shot dead while he attended Christmas day mass at a Roman Catholic church in the eastern town of Batticaloa.
Raviraj had earlier criticised the security forces for killing at least 65 civilians Wednesday when they shelled a rebel-held area and hit a public building where hundreds of displaced persons were sheltering.
In the neighbouring district of Trincomalee, six guerrillas were killed when the military blasted two rebel boats in a fresh clash Friday, the defence ministry said.
The confrontation came barely 12 hours after two separate sea battles between the navy and the Tamil Tigers off the island's northern Jaffna peninsula, where the guerrillas sank two government gunboats.
The Liberation Tigers of Tamil Eelam (LTTE) said they had captured four sailors and killed about 25 in the two battles.
The guerrillas also said they had captured a large haul of weapons from a gunboat before sinking it.
The surge in violence came as the United Nations and the United States severely criticised Wednesday's shelling by the Sri Lankan army.
"The UN condemns in the strongest possible terms the shelling by the security forces of the government of Sri Lanka on defenceless civilians sheltering in Kathiravelli school," the UN office here said in a statement.
"Equally disturbing are reports that the rebel Liberation Tigers of Tamil Eelam prevented some 2,000 civilians from fleeing to safety."
Truce monitors reported rockets fired into two school buildings housing a large number of Tamil civilians displaced by the latest fighting in this three-decade long conflict.
The conflict has claimed more than 60,000 lives as the rebels fight for a separate homeland for minority Tamils in this Sinhalese-majority country.
The government has expressed "regret" for Wednesday's bombardment, but also blamed the Tigers for using human shields.
"We have growing concern," US Assistant Secretary for South and Central Asian Affairs Richard Boucher told reporters in New Delhi.
"There is news this morning of the killing of one of the Tamil politicians. It is very, very important for both sides to respect the cease-fire. The only way to achieve what they want is through negotiations.
"They are clearly not going to get a solution through violence," Boucher warned.