(Adds air raids, details)

By Ranga Sirilal

COLOMBO, May 16 (Reuters) - A suicide bomber crashed his motorbike into a police bus in the Sri Lankan capital on Friday, killing 10 people, and authorities said Tamil Tiger rebels were to blame.

Doctors at the National Hospital in Colombo said eight policemen and two civilians were killed and more than 85 people wounded in the blast in a commercial quarter of the capital.

The area is a high security zone containing Sri Lanka's presidential office and military headquarters.

The military blamed the attack on the Liberation Tigers of Tamil Eelam (LTTE), who want an independent state in the north and east of the island. The Tigers, who routinely deny involvement in such attacks, were not immediately available for comment.

"The target was a police bus and a motorbike crashed into the police bus," said military spokesman Brigadier Udaya Nanayakkara.

President Mahinda Rajapaksa condemned the attack and appealed for calm.

"With this bomb attack on a busy street, the LTTE has once again demonstrated to the world its total commitment to violence and terror to achieve its separatist goals in Sri Lanka, and its absolute contempt for democracy and human rights," he said in a statement.

Sri Lankan television showed the shattered side of the bus and other damaged vehicles, and casualties being rushed to hospital in private vehicles and taxis.

Fighting between government forces and the Tigers has intensified since the government formally pulled out of a six-year-old ceasefire pact in January, though a renewed civil war has been raging since 2006.

Air force fighters carried out raids on a Tamil Tiger runway in rebel-held Kilinochchi in the far north on Friday evening.

"Fighter jets bombed Iranamadu runway at 8.00 p.m. and pilots say the target was taken accurately," said air force spokesman Wingcommander Andrew Wijesuriya.

On Friday morning, jets bombed a rebel sea Tiger base in the north for a second day running, the military said, adding ground troops had killed 33 rebel fighters over the past two days.

The LTTE has not commented on the latest clashes and independent confirmation of battlefield casualties is not possible because of lack of access. Each side is known to exaggerate the other's losses.

According to a compilation of military data, about 360 rebels have been killed in the fighting in May with the loss of 41 soldiers. An estimated 70,000 people have been killed in the 25-year civil war.

Analysts say the military has the upper hand in the latest phase of the conflict because of superior air power, strength of numbers and swathes of terrain captured in the island's east. But they still see no clear military victory on the horizon.

The Tigers regularly hit back with roadside bombs and suicide attacks, increasingly against civilians, experts and the military say, deterring tourists and worrying investors. (Editing by Andrew Roche)

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