"One of the major human rights shortcomings in Sri Lanka is rooted in the absence of reliable and authoritative information on the credible allegations of human rights abuses," she said at the end of a five-day mission to the country.
"In the context of the armed conflict and of the emergency measures taken against terrorism, the weakness of the rule of law and prevalence of impunity is alarming."
Mahinda Samarasinghe, the Sri Lankan human rights minister, who attended a news conference with Arbour, dismissed any type of monitoring mission.
Instead, Samarasinghe said, the government was willing to work with Arbour's office and others in sharing technical expertise and training local staff to face human rights challenges.
Arbour said the government told her of its initiatives to address allegations of human rights abuses, but "there has yet to be an adequate and credible public accounting for the vast majority of these incidents".
The civil war has killed an estimated 70,000 people since it began in 1983. A ceasefire was reached in 2002 to pave the way for a peace deal between the government and Tamil separatists fighting for a homeland for their minority, but it fell apart nearly two years ago.
The renewed fighting has killed an estimated 5,000 people. Human Rights Watch said in August that more than 1,100 abductions or "disappearances" were reported between January 2006 and June 2007, many of them blamed on the government and its armed allies.