A security alert was in place across Sri Lanka Sunday after Tamil Tiger rebels vowed to retaliate for air attacks, following a day of fierce battles on land and sea that left more than 50 people dead.

A police water bowser was blown up in a Claymore mine attack in the north-central district of Anuradhapura, killing three constables, the military said.

A military spokesman said they believed the Liberation Tigers of Tamil Eelam (LTTE) carried out the bombing.

Also Sunday the Tigers and a breakaway faction of rebels clashed in the island's east leaving at least six guerrillas dead, military officials said, citing intercepts of rebel radio communications.

Already tight security was further strengthened in the face of escalating violence in northern and eastern regions on Saturday that saw vicious fighting between troops and rebels.

"We have made sure security is tight," inspector general of police, Chandra Fernando said Sunday. "We are seeking public co-operation to track down any suspicious activity."

Fernando said the LTTE had deployed a new type of sea mine along the northwest coast, but five divers laying the mines were arrested. One died after committing suicide by swallowing a cyanide capsule.

The arrests came as the LTTE warned that the country will be pushed back into a "fatal war" if the military resumes air strikes.

Fighter jets bombed suspected Tiger positions on Thursday and Friday as a deterrent against more attacks following Thursday's blast on an overcrowded bus in which 64 people were killed.

On Saturday at least 52 people were killed as fierce land and sea battles erupted.

The leader of the LTTE's political wing, SP Thamilselvan, denied Tiger involvement in Thursday's bus attack.

"We categorically deny the accusation that the Tigers attacked the civilian passenger bus," he said in remarks published on the pro-rebel Tamilnet.com website.

The military has brushed aside rebel denials and carried out air attacks as well as artillery strikes against Tiger positions.

There have been no reports of casualties and Tamilnet reported on Friday that two out of six bombs dropped by the planes did not explode.

The LTTE fought a major sea battle against navy patrol craft along the northwest of the island on Saturday, killing at least 12 sailors and destroying three boats, officials said.

The defence ministry said the navy and the air force beat back the Tiger offensive by sinking eight Tiger craft and killing 30, but the rebels said they suffered only two people wounded.

Six civilians were also killed in the crossfire while 28 were admitted to hospital with gunshot and shrapnel injuries. Both the government and the rebels blamed each other for the civilian casualties.

Three more sailors were killed when their boat capsized while rushing to the aid of colleagues under attack.

Saturday's incidents were along a 185 kilometer stretch of the northwest coast. The clashes marked a further escalation of violence despite a February 2002 truce.

International condemnation of the bus attack was followed by calls for both the Tigers and the Sri Lankan Government to return to the negotiating table.

The European Union, which last month banned the Tigers as a terrorist organization, urged the parties on Saturday to negotiate a settlement to the conflict, which has claimed more than 60,000 lives since 1972.

Peace broker Norway this month failed to arrange face-to-face talks between the Colombo government and the LTTE in Oslo.

(China Daily June 19, 2006)