"The dangerous escalation of the conflict in recent days is a direct consequence of killings being allowed to run unchecked," said Philip Alston, UN's special rapporteur on extrajudicial, summary or arbitrary executions.
He told the government and the Liberation Tigers of Tamil Eelam (LTTE) to take urgent measures to end "political killings" and to protect human rights.
"The purpose of these killings has been to repress and divide the population for political gain," Alston said in a report that blamed both sides for a series of killings in December.
"Today many people -- most notably, Tamil and Muslim civilians -- face a credible threat of death for exercising freedoms of expression, movement, association, and participation in public affairs."
He expressed "deep concern" about the widespread murders that have continued since his November-December visit when he met government and rebel representatives.
Violence has increased this week after a suicide bomb attack Tuesday on the army chi