Burundi has banned three UN investigators from entering the country after they accused the government of gross human rights violation
The investigators said in a report last month that thousands of people had been tortured, suffered sexual abuse or disappeared during political violence.
The decision comes days after Burundi announced plans to withdraw from the international criminal court (ICC).
A letter signed by foreign affairs minister Alain Aime Nyamitwe said Pablo de Greiff of Colombia, Christof Heyns of South Africa, and Maya Sahli-Fadel of Algeria were no longer welcome in Burundi.
Speaking in New York on Monday, UN spokesman Stephane Dujarric urged Burundi to continue to co-operate with the investigators.
Burundi descended into political turmoil in April 2015 when president Pierre Nkurunzinza began his bid for a third term.
Since then, more than 500 people have died and at least 270,000 have fled the country.