The United Nations envoy to Burundi announced on Wednesday that he planned to step down from the post he has held for two years, amid concerns over the impartiality of elections set for 2020.

"The situation in Burundi remains tenuous," Michel Kafando told the Security Council, lamenting "growing political intolerance and attacks on civic and political freedoms."

Kafando said he hoped the May 2020 elections would be "transparent."

"Poorly organized and contested elections are always a source of conflict," he warned.

Several diplomats have told AFP they are concerned the vote will be unfairly organized, potentially re-igniting conflict in the country.

Kafando pointed out that Burundi's security situation has improved and that the government has put in place a "framework for political dialogue."

But he complained about the lack of progress of the inter-Burundian dialogue guided by the East African Community, and warned about the ongoing deterioration of the socio-economic situation.

Kafando, who served as interim president of Burkina Faso from 2014 to 2015, told the Security Council he had already informed UN Secretary General Antonio Guterres of his resignation in June.

The month prior, Burundi had threatened to cut ties with Kafando, according to UN diplomats, leading the Security Council to abruptly scrap a meeting on the situation in the country.

Burundi President Pierre Nkurunziza has said he will not stand in next year's elections, amid a crisis that began when he announced his bid for a controversial third term in April 2015. He was re-elected three months later.

Violence related to the crisis has left at least 1,200 people dead and displaced more than 400,000 between April 2015 and May 2017, according to estimates by the International Criminal Court, which opened an investigation.