(Adds background, comments by U.S. diplomat, Amnesty and Sri Lankan rights chief)
By Stephanie Nebehay
GENEVA, March 27 (Reuters) - The United Nations on Thursday launched an inquiry into war crimes allegedly committed by both Sri Lankan state forces and Tamil rebels during their conflict that ended in 2009, saying the government had failed to investigate properly.
"The international community has become increasingly concerned by the continued lack of progress in achieving reconciliation, justice and accountability for serious allegations of violations of international human rights law and international humanitarian law," Paula Schriefer, U.S. deputy assistant secretary of state, told the U.N. Human Rights Council (UNHRC).
Sri Lanka, which rejected the resolution, has been under international pressure to deal with war crimes allegedly committed in the final stage of the 26-year conflict in which the army defeated separatist Tamil Tiger rebels.
Sri Lanka's ambassador, Ravinatha Aryasinha, told the talks on Thursday that the Indian Ocean island nation had made tangible progress in addressing accountability. He said parallel processes would be "counter-productive".
"Sri Lanka categorically and unreservedly rejects this draft resolution as it challenges the sovereignty and independence of a member state of the U.N....and is inimical to the interests of the people of Sri Lanka," Aryasinha said in a speech before the vote.
The head of Sri Lanka's human rights council said the UNHRC report could lead to sanctions on the country by the U.N. Security Council, including a freeze of bank accounts and bans on travel by Sri Lankan leaders.
"That is where the problem will start," said Prathiba Mahanama. "But we still have an opportunity to do things in a way to prevent a report reaching the Security Council."
A total of 23 states voted in favour, 12 against and 12 abstained. States that voted against included China and Pakistan, while Sri Lanka's neighbour, India, was among those that abstained.
Launching an international inquiry into wartime abuses and monitoring abuses since then, including the harassment of journalists, lawyers and activists, was a priority of the U.S. delegation during the four-week council session.
The investigation will be led by U.N. human rights chief Navi Pillay, a Tamil of Indian origin who is from South Africa.
Amnesty International said the inquiry would bring hope for thousands of victims of abuses in Sri Lanka.
(Additional reporting by Shihar Aneez in COLOMBO; Editing by John Chalmers and Angus MacSwan)