By Ranga Sirilal

COLOMBO (Reuters) - Sri Lanka released 1,800 rehabilitated Tamil Tiger fighters on Friday, as part of what it says are efforts to reconcile ethnic differences after a quarter of a century of war.

The government argues its attempts to reintegrate separatist Tamil Tigers, who were defeated in May 2009, must be given time to take effect and says it is on guard to block Western moves to push forward a war crimes probe.

President Mahinda Rajapaksa presided over the ceremony to release the former Tiger combatants, who have gone through job and language training before being released from government custody.

A Western-led push for a war crimes investigation, backed by rights groups and spearheaded mainly by countries with large Sri Lankan Tamil populations such as Canada, gathered steam at the recently concluded U.N. Human Rights Council sessions.

U.N. Secretary-General Ban Ki-moon sent a report he commissioned, highly critical of the government, to the council, meaning member states could take its contents up and potentially force Sri Lanka to submit to a war crimes probe.

"Even though we managed to clamp down and defeated several attempts including that of Canada, we know that the threat is not over," said Plantation Minister Mahinda Samarasinghe, who has a special brief to handle the war crimes issue at the UNHRC.

At the UNHRC meeting, Canada brought a resolution to suggest that the next session in March should discuss a war crimes report by Sri Lanka's Lessons Learned and Reconciliation Commission (LLRC), due to be presented to Rajapaksa on November 15.

OTTAWA VS COLOMBO

Canada, home to the largest population of Sri Lankan Tamils outside the Indian Ocean island, criticized the government's human rights record for the first time on Thursday, which could lead to a confrontation at the 54-member Commonwealth summit of former British colonies in Australia next month.

It also has threatened to boycott the next Commonwealth summit, which Sri Lanka is due to host in 2013.

"There can be many fresh attempts and we are now reorganizing and getting ready to face them with the support of our friendly nations," Samarasinghe said.

Those include China and Russia, both U.N. Security Council members with the power to veto any moves against Sri Lanka there. That has led Sri Lanka's critics to attempt to bring about a probe in the rights council.

The report commissioned by Ban, which Sri Lanka says is biased, inaccurate and mirrors propaganda from Tiger supporters, accused government forces of killing thousands of civilians indiscriminately.

Diplomats say the war crimes pressure is mainly to force Rajapaksa's government to create a durable political compromise with Sri Lankan Tamils, to forestall a reignition of the war that stemmed from decades of post-independence moves that sidelined them politically.

Sri Lanka has said its troops used only necessary and lawful force against the Tamil Tigers, a separatist group that was on the terrorism lists of 30 nations for its use of suicide bombers and child soldiers, among many other rights violations.

(Writing by Bryson Hull; Editing by Rosalind Russell)