By Ranga Sirilal and Shihar Aneez
COLOMBO, July 17 (Reuters) - Sri Lanka's president has extended the terms of a commission investigating missing people and possible war crimes in the country's 26-year civil war, bringing in foreign experts for the first time to advise on the inquiry, the government said on Thursday.
Mahinda Rajapaksa's move, contained in a document issued this week and obtained by Reuters on Thursday, comes as international pressure intensifies on Sri Lanka to investigate the final stages of the war in 2009 to crush ethnic minority Tamil separatists.
Three legal experts - two Britons and a U.S. national - were appointed as part of an international advisory panel linked to the presidential commission set up last year to conduct the inquiry.
But a government spokesman made clear that whatever recommendations the panel might make, the island nation's authorities were free to accept or reject them.
"This is just to advise the commission. We can decide whether to accept the advice or to set it aside," government spokesman Keheliya Rambukwella told a news conference.
"It is to double check what we have found so far and to get some international backing. It is not a re-correction."
The United Nations has launched an inquiry into war crimes allegedly committed by both Sri Lankan state forces and Tamil rebels in the final months of the conflict, saying the government has failed to investigate properly.
Sri Lankan authorities reject the allegations on grounds they amount to interference in domestic affairs and say they are addressing issues of accountability.
The military dismisses any notion that war crimes were committed in the final offensive against the separatists of the Liberation Tigers of Tamil Eelam (LTTE) or that civilians in Sri Lanka's Tamil-dominated north were targeted.
A senior government official, speaking on condition of anonymity, said the president was allowing the participation of foreigners to ease pressure from abroad.
"The government is under pressure. That's why it has had to resort to these measures," the official said.
The presidential document said the commission's mandate was to investigate "facts and circumstances that led to the loss of civilian life" in the final stages of the conflict and whether any individuals or groups should bear responsibility.
The three foreign experts named to the panel are Sir Desmond de Silva, a British lawyer and former U.N. war crimes prosecutor in Sierra Leone, Sir Geoffrey Nice, who was part of the International Criminal Tribunal for the Former Yugoslavia and David Crane, chief prosecutor of the Special Court for Sierra Leone from 2002 to 2005.
(Reporting by Ranga Sirilal and Shihar Aneez; Editing by Ron Popeski)