COLOMBO - Sri Lanka is to hold an international seminar starting Tuesday to share lessons on its bloody defeat of Tamil rebels, but major nations have declined to send delegations amid war crime allegations.

The island's government said the massive military offensive that finally defeated the Tamil Tigers two years ago was a victory over terrorism, and that other countries could benefit from studying its success.

But New York-based Human Rights Watch has urged a boycott of the seminar, entitled "Defeating terrorism, Sri Lankan experience", over claims that thousands of civilians died in military bombing of so-called no-fire zones.

Of the 54 countries invited, about 12 have declined, officials told AFP.

Among the nations not sending delegations are the United States, Britain, Australia, France, Japan and Switzerland, who had all pressed Sri Lanka to investigate the alleged war crimes.

China, the island's key arms supplier, is co-sponsoring the conference, which will also be attended by a delegation from Russia, another strong ally of President Mahinda Rajapakse.

"The Sri Lankan approach that succeeded in defeating the (rebels) was the result of learning from years of fighting insurgency, strong political will and harnessing popular support," a statement from the seminar said.

"The panel presenting the Sri Lankan experience will share their knowledge on counter-insurgency and enumerate contributory factors in militarily defeating the LTTE (Liberation Tigers of Tamil Eelam)."

A panel of UN experts last month put together what they called "credible allegations" of government forces shelling civilians, bombing hospitals and killing surrendering rebel leaders.

"Sri Lanka's self-proclaimed 'model' of counter-insurgency included repeatedly shelling civilians, targeting hospitals, and trying to prevent the world from finding out about it," Human Rights Watch said.

HRW's Asia Director Brad Adams said no rights-respecting nation should take part in the conference and described the three-day meet as an attempt to whitewash abuses.

The government has rejected all calls for a war crimes probe into the allegations and said it wanted to end the civil war to build national unity in the ethnically divided country.

China and Russia have helped Sri Lanka stave off censure at the UN, including the Human Rights Council in Geneva.

The Tamil Tigers, who fought for an independent homeland for the island's ethnic Tamil minority, were also accused of holding civilians as human shields and killing those who tried to escape.

The Tigers were once seen as the world's most effective guerrilla outfit.

They controlled a third of the island's territory in the mid-1990s and deployed suicide bombers to assassinate high profile targets.

When a truce collapsed in 2006, Colombo brushed off calls for more negotiations and unleashed a no-holds-barred offensive that killed the last rebel leaders in May 2009.

President Rajapakse on Friday marked the second anniversary of defeating the Tigers by again rejecting any war crimes probe.

"Our troops went to the battle front carrying a gun in one hand, the Human Rights Charter in the other, food for the innocent displaced on their shoulders, and love of their children in their hearts," Rajapakse said.

He is not due to address the seminar, but his youngest brother, defence secretary Gotabhaya Rajapakse, is set to deliver the keynote address on Tuesday.