Report to be issued on Wednesday after six-month delay

* UN rights boss: "Findings are of the most serious nature"

* Government has sought reconciliation with minority Tamils (Adds quote, details, background)

GENEVA: The United Nations will release a long-delayed report on Wednesday on war crimes committed during Sri Lanka's civil war that will demand perpetrators face justice, the UN High Commissioner for Human Rights said on Monday.

The global body originally meant to release its findings in March on atrocities committed by both government and "Tamil Tiger" rebels, but agreed to hold off for six months to let a new government look into why suspects had not been prosecuted.

UN High Commissioner for Human Rights Zeid Ra'ad Al-Hussein praised the new government's efforts on Monday but said it was time to press on with the report.

"Its findings are of the most serious nature," he told the main UN rights forum in Geneva.

"This council owes it to Sri Lankans - and to its own credibility - to ensure an accountability process that produces results, decisively moves beyond the failures of the past and bring the deep institutional changes needed to guarantee non-recurrence," he said.

According to an earlier UN report, around 40,000 ethnic minority Tamils were killed in a final offensive ordered by former president Mahinda Rajapaksa to end the long rebellion by the Liberation Tigers of Tamil Eelam in 2009.

The UN council does not have the power to order prosecutions but its report is expected to push for a judicial process, backed by international monitors, that could implicate officials at the highest levels during Rajapaksa's rule.

Rajapaksa himself has also called for accountability.

A new coalition government formed by Rajapaksa's successor, President Maithripala Sirisena, has sought reconciliation with minority Tamils and promised a domestic judicial process, but has stopped short of spelling out how it will work.

Prime Minister Ranil Wickremesinghe, making his first foreign trip since last month's election, was due to arrive in India later on Monday.

He was expected to ask Indian Prime Minister Narendra Modi Modi to back a proposed US resolution in support of a domestic process, government and independent sources in Sri Lanka said, in the hope that India can use its influence to overcome doubts in the Tamil community that it can be fair and impartial.

India, which has close links to the Sri Lankan Tamil community, abstained last year from a resolution calling for an independent international investigation into alleged war crimes.