Sri Lanka's army said Wednesday it agreed for the first time to probe war crime charges against its troops and investigate allegations of prisoner executions made in a British TV documentary.

Army chief Jagath Jayasuriya appointed a court of inquiry to investigate charges that troops were responsible for killing civilians and prisoners in the final stages of their war against Tamil rebels in 2009, an army statement said.

The dramatic U-turn from the security establishment came two days after the US warned that the Indian Ocean island would face censure at next month's UN Human Rights Council meeting in Geneva.

Lieutenant General Jayasuriya asked a five-member panel of officers to investigate allegations against the force, including that it executed prisoners as claimed in a documentary by Briton's Channel Four television channel.

The probe is a major shift for Sri Lanka's armed forces, which had insisted that it did not kill a single civilian while crushing the separatist Tamil Tiger rebels in a no-holds-barred offensive that ended in May 2009.

The army said the inquiry was ordered after the government's own Lessons Learnt and Reconciliation Commission (LLRC), a panel which probed why a 2002 truce collapsed, said civilians had perished as a result of military action.

"If there is a prima facie case disclosed against any person from the evidence led before the Court of Inquiry, a General Court Martial will be convened to try the alleged offenders," the army said.

It said an inquiry into observations of the LLRC and the "Channel 4 video footage irrespective of its authenticity or otherwise, is now in full progress."

Human rights groups have said up to 40,000 civilians were killed in the final months of the government's military campaign.

The UN estimates some 100,000 people perished during Sri Lanka's 37-year ethnic conflict which ended with the wiping out of the entire military leadership of the Liberation Tigers of Tamil Eelam.