Burundi’s President Pierre Nkurunziza will not abandon his bid for a third term even as the United Nations warns on increased violence by the pro-government militia.

"This decision is non-negotiable," government spokesman Philippe Nzobonariba said in a radio broadcast Tuesday, dismissing opposition demands that President Nkurunziza steps down. He also said the electoral commission's proposal to delay the presidential poll until July 15 was the final time elections would be postponed.

The national independent electoral commission (CENI) had, on Monday, set the presidential election to take place on July 15 after parliamentary polls are held on June 26.  The commission said the dates had been proposed in accordance to an East African Community Heads of State recommendation that elections be postponed for at least 45 days.

However, the opposition questioned CENI’s legitimacy owing to a lack of quorum after two officials resigned and fled to Rwanda, leaving only three commissioners.

READ: Burundi electoral body picks new poll dates

The UN human rights chief on Tuesday warned that the Imbonerakure, militia attached to the ruling party, were spreading fear throughout the country.

Threats of violence, alleged executions and abductions "could tip an already extremely tense situation over the edge," cautioned UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein.

“Every day, we receive 40 to 50 calls from frightened people across the country pleading for protection or reporting abuses,” he said.

Refugees’ accounts

Accounts by Burundian refugees tell of attacks carried out by the Imbonerakure in several provinces as well as in the capital city, Bujumbura, Mr Hussein added.

He cited the example of a 19-year-old refugee from Makama Province who said his father had been stabbed to death for refusing to join the ruling party.

A female refugee from Rugongo town in Citiboke Province told UN staff members that her husband had been beaten and abducted by Imbonerakure members who demanded to know why he was not taking part in meetings organised by the ruling party. The man is said to be missing since.

“These reports are truly chilling, particularly in a country with a history like Burundi’s,” Mr Hussein said.

An estimated 300,000 Burundians died in the course of a civil conflict that formally ended 10 years ago.

"We have been receiving consistent testimonies indicating that Imbonerakure members operate under instructions from the ruling party and with the support of the national police and intelligence services, who provide them with weapons, vehicles and sometimes uniforms," Mr Hussein said.

"If State authorities are indeed colluding with a violent lawless militia in this manner, they are gambling with the country’s future in the most reckless manner imaginable.”

The UN chief also called on opposition leaders to prevent violence on their side of Burundi's political divide.

“While so far there have been very few acts of violence committed by opposition elements, there are signs of increasingly coercive efforts to push people into actively supporting the opposition,” he said.

“The last thing Burundi needs after a decade of gradual and largely successful peace-building is to be catapulted back into civil war because of a small number of people’s ruthless determination to retain or gain power at any cost,” the high commissioner added. 

The Burundi parliamentary and presidential elections were earlier scheduled for June 5 and June 26 respectively.

Around 40 people have died and scores more have been injured in protests that began when Nkurunziza announced in late April that he would stand again, after Burundi's constitutional court gave him the green light.

Additional reporting by AFP