The Northern Provincial Council wants to conduct its own internationally supervised count of the dead and missing civilians to back its claim.
The moves come ahead of U.N. Human Rights Council sessions in March where member countries will review Sri Lanka’s post-war accountability process.
The U.N. human rights body has passed two resolutions calling on Sri Lanka to conduct its own investigations into war crimes allegations against government troops and the now-defeated Tamil Tiger separatist rebels who fought a quarter-century war that ended in 2009.
Countries like the United States and Britain have warned that the South Asian island nation could face an international inquiry if it fails to do so.
A U.N. report previously said some 40,000 Tamil civilians died, mostly in government attacks in the final months of the fighting.
Sri Lanka has repeatedly denied it deliberately targeted civilians, and last month it took a census of the dead, wounded and missing civilians. Its report is expected to be out in time for the U.N. session.
The provincial council resolution also urged that investigations on a mass grave found recently be held with the help of U.N expertise. At least 48 human skeletons have been found in the grave in northern Mannar district.
The Sri Lankan government has been accused of deliberately targeting hospitals and blocking food and medicine as a war strategy. It also has been accused of deliberately undercounting the number of civilians who were trapped in the fighting zone from which authorities evicted U.N and other aid workers.