By Ranga Sirilal and Simon Gardner

COLOMBO, May 24 (Reuters) - Tamil Tiger boats attacked and infiltrated a Sri Lankan naval base off the island's far north on Thursday, killing several sailors, while a rebel roadside bomb targeted an army bus near Colombo port wounding seven, the military said.

The Tigers claimed they killed 35 sailors during the predawn raid on an island off the army-held northern Jaffna peninsula, but the military dismissed it as propaganda and said less than 10 sailors died in the attack.

"The Tigers have attacked a small naval detachment on Delft island in Jaffna. Less than 10 sailors are dead," said military spokesman Brigadier Prasad Samarasinghe. "We didn't even have 35 people there. That's totally false propaganda."

In a separate incident, bomb squad officials said a parked motorbike packed with explosives detonated as an army bus drove past near the gate to Sri Lanka's main port in the capital, blowing out the windows of the military vehicle.

However they said they did not believe it was a suicide attack, as military officials initially reported.

A Reuters witness at the scene saw two charred motorbikes on the ground near the bus, but said the bus appeared to be intact. He said he could see blood on the floor of the bus.

Hospital officials said seven people, including four military personnel, were wounded in the blast, but said no-one was killed. Two of the wounded were in a serious condition.

The Tigers said they had no involvement in the blast, which bore the hallmarks of a series of attacks on the military in recent months that analysts say the rebels were definitely behind.

"SUCCESSFUL ATTACK"

"We were not involved," Tiger military spokesman Rasiah Ilanthiraiyan said by telephone from the rebels' northern base of Kilinochchi, instead touting the naval base attack as a success.

"We had the entire navy base under our control and found 35 navy personnel dead bodies during a search operation," he added. "Four of our Sea Tigers were killed in action."

He said the rebels' naval wing later left the island to return to an area of the mainland they control, and denied the military's claim that it sank a Tiger suicide boat.

"It was a surgical operation. It was not a real estate-grabbing operation," Ilanthiraiyan said. "We can't stay there without supplies. It was an attack to destroy their assets."

The attacks were the latest in a series of land and sea clashes and ambushes in recent months amid a new chapter in a two-decade civil war that has killed nearly 70,000 people since 1983.

Fighting is now focused on the north after the military captured the Tigers' eastern stronghold in recent months, but analysts say there is no clear winner on the horizon and fear the protracted conflict could rumble on for years.

President Mahinda Rajapaksa's government has pledged to destroy the Tigers' militarily, while the rebels have vowed to fight on a for an independent state in the north and east.

The Tigers have also warned they will launch more air raids by their homegrown air force after a series of daring night-time attacks that have sent jitters through the Sinhalese-majority south, including the investment community.

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