"At the invitation of the government of Kenya, the United Nations Special Rapporteur on extrajudicial, summary or arbitrary executions, Professor Philip Alston, has arrived in the country," a U.N. statement released on Sunday said, according to the Reuters.

The statement said: "His responsibilities include reporting on alleged killings and the underlying causes that might have prevented effective legal action to prosecute and punish those responsible.”

The issue of justice over last year's killings is straining the coalition government set up to end the worst electoral violence in Kenya since independence from Britain in 1963.

Unity government leaders Kibaki and Prime Minister Raila Odinga -- whose dispute over the election touched off the crisis -- failed last week to push through parliament the creation of a special local tribunal to try the alleged perpetrators.

Alston was due to visit the capital Nairobi and western towns such as Kisumu and Eldoret, the scenes of bloodletting during last year's post-election violence in which at least 1,300 people were killed.

He was expected to investigate not just deaths during the post-election violence but all allegations of brutality and summary executions by Kenyan security forces.

Police death squads are accused of killing some 500 alleged members of the Mungiki criminal gang.

Alston will also go to the western Mt. Elgon region that was the scene of an offensive against rebels. Local rights groups said Kenyan security forces tortured more than 4,000 people in an indiscriminate offensive in the remote area.

Activists said the systematic abuses -- including crawling on barbed wire, pouring water into mouths and forcing victims to whip each other -- was the worst wave of torture in Kenya under the government of President Mwai Kibaki, in power since 2002.

Based on information obtained during the visit, Alston will present a report containing his conclusions and recommendations to a forthcoming session of the Human Rights Council.