(Adds air strikes, bourse fall, updates death toll)

By Ranga Sirilal

COLOMBO, Oct 22 (Reuters) - The Tamil Tigers' air wing bombed a north Sri Lanka air force base before dawn on Monday, the military said, while the Tigers said suicide fighters mounted their biggest ground assault since the two-decade civil war began.

The rebel air strike in the north-central district of Anuradhapura comes months after the Tigers' first ever air attacks using light aircraft smuggled into the country in pieces, and as near daily land, air and sea clashes occur.

Nine servicemen were killed inside the base and 20 others were wounded in the attack, while four crew aboard a helicopter gunship, scrambled to search the area, were killed when it crash-landed several kilometres away, the military said.

The crash was due to technical reasons and not rebel or friendly gunfire, it added.

Traders said the two-pronged attack was partly to blame for losses on the Colombo stock exchange, which closed provisionally 0.43 percent lower as worried small investors sold shares.

Twenty Tiger fighters were killed during a gunbattle at the base, the government said. There were no independent accounts of what happened or how many people were killed. The Tiger aircraft escaped.

"We have found 20 bodies of LTTE cadres inside the base and we are doing a search operation to see if there are any more," government defence spokesman Keheliya Rambukwella said. "This will not deviate us from the fight against terrorism."

An air force spokesman said two MI-24 helicopters and a training aircraft parked at the base were damaged in the attack. Residents in Anuradhapura, where police imposed a curfew, could still hear gunfire by early afternoon.

A search operation was under way around the air force base, one of Sri Lanka's largest. Air force fighter jets retaliated with bombing raids on a suspected rebel air strip at the town of Iranamadu, inside Tiger territory.

The Tigers' air wing of light aircraft bombed oil installations and an air base adjacent to the island's only international airport earlier this year. The Tigers have warned more such attacks could follow.

MORE ATTACKS LOOM?

"It was a combined operation of our air force and land troops and the target was the Anuradhapura air base, which functions as the logistics (base for the north)," rebel military spokesman Rasiah Ilanthiraiyan said by telephone from the Tigers' northern de facto capital of Kilinochchi.

"We have not ruled out the potential of more similar attacks in the future."

He said 21 Black Tiger suicide fighters took part in the ground assault.

"It is the biggest operation of the Black Tigers since 1987," he added. "We damaged eight of their aircraft at the base."

The Tigers e-mailed photographs of the suicide fighters, dressed in black and grey trademark Tiger-stripe fatigues, pictured with shadowy rebel leader Velupillai Prabhakaran.

Monday's attack in the north, where renewed civil war is now focused after troops drove the Tigers from bastions in the east of the island, comes after the military said dozens of Tigers were killed in heavy fighting in the north last week.

An estimated 5,000 people have been killed since early last year, taking the death toll since the conflict erupted in 1983 to around 70,000.

The Tigers seek to carve out an independent state in the north and east. The government rules that out and has instead vowed to evict the rebels from all territory they control.

While the government has had the upper hand in recent months, analysts say there is no clear winner on the horizon and fear the conflict could rumble on for years.

Counter-terrorism experts say the only hope is for both sides to reach a long-elusive political settlement. (Additional reporting by Shihar Aneez)

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