The Sri Lankan government has expressed its concern over UN secretary general's decision to set up a panel to look into alleged human rights abuses.
There have been several allegations that both the army - and Tamil Tigers rebels who they routed last year - committed crimes at the end of the war.
Ban Ki-moon's spokesman said the three-man panel would advise on how to deal with alleged perpetrators.
About 7,000 civilians died in the last five months of the war, the UN says.
Rights groups accuse both sides of war crimes - a claim which has been denied.
"Sri Lanka regards the appointment of the Sri Lanka-Panel of Experts as an unwarranted and unnecessary interference with a sovereign nation," a foreign ministry statement said.
"This interference, moreover, has potential for exploitation by vested interests hostile to the process of reconciliation taking place in the country," it said.
Sri Lanka's media minister, Keheliya Rambukwella, told a local news website that the government was "concerned" that Ban Ki-moon, as an outsider, had appointed this panel of human rights advisers, given that Sri Lanka recently announced its own team looking at reconciliation issues.
"We will take appropriate action when it comes to the next stage," the minister said.
Mr Ban's three-member panel is headed by a former attorney-general of Indonesia, Marzuki Darusman.
He was part of an international team which was appointed to observe proceedings on a previous Sri Lankan commission on atrocities but which resigned saying that commission did not meet basic minimum standards.
The other two members are South African human rights lawyer Yasmin Sooka and Steven Ratner, a US international law expert.
The BBC's Charles Haviland in Colombo says that the UN's new panel is advisory in nature and is not an investigation commission.
Our correspondent says that the domestic Sri Lankan one, appointed a few weeks ago, also has limited and quite vague terms of reference.
The 37-year conflict ended last year with the defeat of the Tamil Tigers in May 2009. The UN estimates 100,000 people were killed.