Sri Lanka announced Thursday an investigation into its military over allegations of rights abuses, following intense international pressure for a war-crimes probe into the island's decades-long ethnic conflict.

President Mahinda Rajapakse said a current government-appointed Commission of Inquiry (COI) would be expanded to probe the military and Tamil rebels over abuses allegedly committed during their war that ended in 2009.

Rajapakse also announced three foreign war crimes experts had been appointed to the commission to act as advisers during the probe.

British lawyers Desmond de Silva and Geoffrey Nice and US law professor David Crane are all former UN war crimes prosecutors.

Under pressure over the military's alleged abuses, Colombo set up the COI last year, but only to trace the thousands of mainly ethnic minority Tamils who went missing during and immediately after the separatist conflict.

In a government decree published Thursday, Rajapakse said the commission would investigate the military's "adherence to or neglect... of laws of armed conflict and international humanitarian law".

The COI is the latest probe initiated by Colombo after several conducted in the past were accused by experts and activists of being a whitewash.

Government spokesman and Information Minister Keheliya Rambukwella said the three foreign experts appointed to the COI will only act as "advisors" and will not have a direct role in the investigations.

"They will be in an advisory capacity," Rambukwella told reporters in Colombo. "That does not mean that we have to take their advice. It is up to us."

Sri Lanka's military concluded in its own investigation that there were no rights abuses committed when crushing separatist Tamil Tiger rebels who controlled a third of the island at the height of their power between 1990 and 1995.

- Over 19,000 missing -

The COI set up in August last year has received 19,284 complaints of missing people, but has so far heard oral evidence only in respect of 786 cases, according to its website.

About 40,000 ethnic Tamil civilians were said to have been killed by government forces in the final months of fighting, a charge Colombo has long denied.

The UN Human Rights Council voted in March to set up an international probe into the allegations, after censuring Colombo for failing to bring the alleged perpetrators to justice.

Rajapakse has said he would not be pressured into cooperating with any international investigation.

The latest decree, published in the government gazette, also said the COI would report on whether the defeated Liberation Tigers of Tamil Eelam (LTTE) is liable for rights abuses under international law.

Colombo had earlier said the entire top LTTE leadership was wiped out during the final stages of the 37-year-old war.

The 1972-2009 conflict claimed 100,000 lives, according to UN estimates.

Sri Lanka has said it needs more time to ensure reconciliation between Tamils and the majority Sinhalese community.

Another panel of foreign experts invited to supervise a presidential inquiry into human rights abuses ended up in disaster in March 2008 when it quit after disagreement with Colombo.