Sri Lanka is to hold an international seminar starting on 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", due to 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.

"I can confirm that Australia was invited but we politely declined," a spokeswoman for Australia's department of foreign affairs and trade told AFP from Canberra.

One Western diplomat in Colombo who declined to be named said that, instead of an attempt by Sri Lanka to celebrate the offensive, "what many in the international community want is a process of accountability".

Sri Lanka's army chief Lieutenant General Jagath Jayasuriya said last week that the boycott call had failed because most nations not sending a military delegation to the seminar would have a diplomat present at the opening session.

The government has rejected all calls for a war crimes probe and said no civilians were killed by its military.

Both 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.

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.