(Adds Amnesty International protest)

By Ranga Sirilal

COLOMBO, April 3 (Reuters) - Sri Lankan troops killed 23 Tamil Tigers in clashes in the east overnight and overran four rebel bases, the military said on Tuesday, as the island's president called for a South Asian anti-terrorism drive.

Monday's clashes in the eastern district of Batticaloa, where troops are seeking to evict the rebels from territory they control, came on the same day suspected Tigers bombed a civilian bus in the island's restive east, killing 16 people, mostly women and children.

"There are 23 confirmed LTTE dead," military spokesman Brigadier Prasad Samarasinghe told a media briefing. "We have captured four bases without confrontation."

The Tigers were not immediately available for comment, but have denied involvement in the bus bombing in the eastern district of Ampara or the slaying of six tsunami aid project construction workers, which the foes each blame on the other.

The latest spree of clashes and attacks have killed more than 50 people since the weekend and come amid near-daily air raids, land and sea battles and ambushes that have killed around 4,000 people in the past 15 months alone.

They also come after the Tigers launched their first-ever air strike on an air base next to the island's international airport north of the capital Colombo last week, which analysts say has added a deadly new dynamic to the protracted conflict.

President Mahinda Rajapaksa's majority-Sinhalese government is pushing on with a declared plan to destroy all Tiger military assets. The Tigers have vowed to fight on for an independent state for minority Tamils in the north and east and have warned of a bloodbath.

REGIONAL ANTI-TERROR PUSH?

In New Delhi for a regional summit, Rajapaksa on Tuesday called on his South Asian neighbours to forge a common anti-terror drive.

"I wish to appeal to this august forum to work jointly on a counter-terrorism strategy for our entire region to defeat terrorism," Rajapaksa told the South Asian Association of Regional Cooperation summit on Tuesday.

"Unless we act collectively as a region, trans-border terrorist groups will find safe havens in other parts of the region," he added.

Analysts say a war that has killed around 68,000 troops, rebels and civilians since 1983 is escalating. But they say the Tigers' retain their strike capability, despite a series of battlefield losses, and see no clear winner on the horizon.

Amnesty International on Tuesday launched a protest against rights abuses on both sides of the conflict to coincide with the cricket World Cup in the West Indies.

The rights group urged supporters around the world to sign cricket balls to be delivered to both sides bearing the message "Play by the rules". The Sri Lankan cricket team is considered one of the top sides in the competition and the campaign has angered the government .

"All parties to the conflict in Sri Lanka are breaking international law by killing civilians, destroying homes and schools, or forcibly disappearing people," Tim Parritt, Amnesty's Deputy Asia Pacific Director, said in a statement, calling for an international rights monitoring mission.

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