Burundi's government defied opposition demands Tuesday for President Pierre Nkurunziza to end a third-term bid for power, as the UN warned the country risked being "catapulted" back into civil war.

"This decision is non-negotiable," government spokesman Philippe Nzobonariba said in a radio broadcast, dismissing opposition demands that the president step down.

The government said the electoral commission's proposal to delay the presidential poll until July 15 was the final time polls would be postponed.

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

EXTREMELY TENSE

The UN human rights chief on Tuesday warned that increasing violence by a pro-government militia, including executions, abductions and torture, was threatening to destabilise further the crisis-wracked central African nation.

"They could tip an already extremely tense situation over the edge," Zeid Ra'ad Al Hussein said in a statement.

"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," he added.

Nearly 100,000 Burundians have sought refuge in neighbouring countries.

SURVIVED COUP

But Nzobonariba said the "vast majority" had "fled because of the terrorist rumours spread by politicians who do not want elections, aided by some foreigners and some non-governmental organisations who want to push Burundi into chaos."

Mr Nkurunziza's opponents say his candidacy is unconstitutional and goes against the 2006 Arusha peace deal that ended 13 years of civil war.

The president survived a coup attempt last month and has since faced down international pressure, including aid cuts, aimed at forcing him to reconsider.

The opposition has rejected proposals for the new election timetable, saying conditions for holding fair polls were not met.

"If things remain as they are, we consider that it will be a masquerade, a parody of elections," opposition leader Charles Nditije told AFP late Monday, demanding the setting up of a new independent electoral commission.

LAWLESS MILITIA

He also called for the disarmament of the ruling party's youth wing, the Imbonerakure, and for Nkurunziza to end his third-term bid for power.

Zeid expressed alarm at repeated allegations "that Imbonerakure members operate under instructions from the ruling party and with support of the national police and intelligence services, who provide them with weapons, vehicles and sometimes uniforms."

"If these claims are even partly true, they indicate an extremely dangerous effort to escalate fear and tension," he warned.

"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 militia has carried out summary executions, abductions, torture, beatings, death threats and other forms of intimidation, according to testimony gathered from 47 Burundian refugees in camps in Rwanda and the Democratic Republic of Congo, the UN statement said.

POLLS PUT OFF

While few violent acts have been committed by the opposition so far, he also urged opposition leaders to "make a huge effort" to rein in any possible violent elements in their midst.

The electoral commission has also suggested delaying parliamentary elections until June 26, and senator elections to July 24.

The parliamentary election had been scheduled to take place on June 5 but was postponed indefinitely on the eve of the vote, while the presidential vote was initially scheduled for June 26.

Civil society leaders have also questioned the impartiality of the key UN mediator, special envoy for the Great Lakes region Said Djinnit.

Vital Nshimirimana said it would be "wise" to appoint another envoy who "inspires confidence in all parties," and calling for stalled talks to restart on "neutral territory" outside the country.