Sri Lanka allowed six more months Monday for an investigation into the end of a three-decade ethnic conflict as rights groups said the commission lacked the power to investigate alleged war crimes.

President Mahinda Rajapakse's office said he had extended the mandate of the Lessons Learnt and Reconciliation Commission (LLRC) until May next year, giving it one full year to complete its report.

Rajapakse appointed a team led by a former attorney general in May this year to study why a 2002 truce broke down and recommend measures to ensure that the country did not slip back to civil war.

"So far more than 100 persons including political activists, social workers, academia, members of clergy, those engaged in conflict resolution and representatives from non-governmental organisations have given evidence," the president's office said in a statement.

New York-based Human Rights Watch, London-based Amnesty International and Brussels-based International Crisis Group have snubbed an invitation to appear before the LLRC, accusing it of a cover-up and lacking credibility.

The panel for its part has said that it must be judged by its performance and not prejudice.

The rights groups have long accused government forces of ordering civilians into a "no-fire zone" and shelling them in the final stages of fighting between government troops and Tamil Tiger separatist rebels in early 2009.

The three groups have said up to 30,000 ethnic Tamil civilians perished in the final months of the conflict, which ended when the rebels were wiped out in May 2009.

Sri Lanka has denied any civilians were killed by its troops and blamed Tamil Tigers for using human shields. Colombo has also rejected an independent international probe and refused to allow UN investigators into the island.

(AFP)