Philip Alston will report his findings to the UN Human Rights Council
The UN special rapporteur on extra-judicial killings, Philip Alston, has begun a 10-day visit to Kenya.
He will talk to human rights groups and eyewitnesses to the violent aftermath of the 2007 presidential election.
Mr Alston will also look into allegations of increased arbitrary killings by security officers.
They include operations against rebel militias in Mt Elgon in western Kenya, and violent encounters against the Mungiki sect in the capital, Nairobi.
His visit comes just days after the government's failure in parliament to establish a local tribunal to charge the perpetrators of the post-election violence which killed 1,500 people.
Last week the chief Kenya negotiator Kofi Annan said he was disappointed by the parliamentary vote and suggested that he might send a list of the suspected ringleaders of election violence to the International Criminal Court.
Widespread clashes broke out after Kenya's current Prime Minister Raila Odinga said presidential results had been rigged in favour of the incumbent, Mwai Kibaki, who he challenged in the polls.
The rivals agreed to share power to bring an end to the violence in February 2008, following weeks of talks led by Mr Annan.