* Pillay says Sri Lanka has failed to investigate
* Colombo rejects her report as arbitrary, political
* UN rights body to address the issue in March (Adds details, Sri Lankan government response)
By Stephanie Nebehay
GENEVA, Feb 25 (Reuters) - The U.N. human rights chief called for an international inquiry into war crimes committed by both sides during Sri Lanka's civil war, saying the government had failed to do its own credible investigation.
In a much anticipated report ahead of a U.N. Human Rights Council debate next month that could order action on the issue, Navi Pillay on Monday recommended an "independent, international inquiry mechanism, which would contribute to establishing the truth where domestic inquiry mechanisms have failed".
U.S. plans to propose a resolution against Sri Lanka at the meeting and Pillay's report, based on her visit to the country last August, add to pressure on the government.
President Mahinda Rajapaksa's administration, in 18 pages of comments as long as Pillay's report, rejected the recommendations as "arbitrary, intrusive and of a political nature".
Many thousands of civilians were killed, injured or remain missing after the 25-year conflict between government forces and separatist Liberation Tigers of Tamil Eelam (LTTE) in the north of the island that ended in May 2009, Pillay said in her report to the Geneva forum.
She said there had been little progress in establishing accountability for "emblematic" wartime crimes, including the January 2006 killing of five students on a beach and the execution of 17 aid workers later that August.
"None of these cases has ... resulted in the perpetrators being brought to justice," she said.
Conclusions of a national Lessons Learnt and Reconciliation Commission have been rejected or ignored, such as its finding that the army was responsible for shelling civilian areas, Pillay said.
Her report did not just direct criticism to the government, but also suggested Tamil Tiger rebels ought to be investigated for any involvement in some of the incidents.
"The High Commissioner (Pillay) is also concerned that legal proceedings have not begun against any LTTE suspect for alleged war crimes or other human rights abuses," the report said.
The report also focused on allegations of abuses after the end of the conflict, although the government says the country is on the path to reconciliation helped by fast economic growth.
It drew attention to concerns that women were vulnerable to sexual harassment and violence when there was a heavy military presence, such as in the northern Tamil heartland, an allegation rejected by Sri Lankan government.
The government said its survey covering 2007-2012 had shown that a majority of the reported incidents of sexual violence in the north were carried out by close relatives or neighbours and "only a very few could be attributed to the security forces".
Pillay also voiced concern at rising attacks by Buddhist monks on minority Muslims and Christians and at harassment of activists, lawyers and journalists.
Pillay said she had received information on 280 incidents of threats and violence against Muslims and 103 against Christians in 2013 alone.
Rejecting the numbers cited, the government said there had been "sporadic incidents focusing on places of worships of all four religions" and it was erroneous to note that there has been a significant surge in attacks against religious minorities. (Additional reporting by Shihar Aneez in Colombo; Editing by Alison Williams)