Burundian opposition parties are discussing uniting behind a single candidate to improve their chances of defeating President Pierre Nkurunziza should he run for re-election in June, an opposition official said on Thursday.

Concerns that Nkurunziza may seek a third term, something his opponents say would be unconstitutional, have created the worst political crisis since 2005, when a 12-year civil war in the east African nation ended.

The constitution and the peace deal that ended that war both say no one should be president for longer than 10 years.

Supporters of Nkurunziza, in power since 2005, argue his first term should not count since he was picked by lawmakers rather than voted in. The president has neither confirmed nor denied he will run for re-election.

"Discussions have started ... to see how we can pick one candidate in order to defeat Nkurunziza's ruling party," said Chauvineau Mugwengezo, spokesman for the Alliance for Democratic Change (ADC-IKIBIRI), a grouping of five opposition parties.

Mugwengezo said former Hutu rebel leader Agathon Rwasa would be the strongest challenger to lead a coalition against Nkurunziza's CNDD-FDD party.

"We all see Rwasa as our leader, but there must be a compromise among our parties," Mugwengezo told Reuters.

If Nkurunziza's opponents united behind a single candidate, it would echo the move of the opposition groups in Nigeria, where the main parties rallied behind Muhammadu Buhari, who beat incumbent Goodluck Jonathan in a March election.

The uncertainties ahead of the June 26 vote, have prompted fears of political violence and several thousand Burundians, have fled across the border to Rwanda in recent days.

UN head Ban Ki-moon has expressed concerns about violence and urged the government to ensure the vote is free and fair.

On Wednesday, the U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein said Burundi was at a crossroads, with one path leading to free and fair elections, the other to "violence and intimidation aimed at subverting democracy for the sake of gaining or maintaining political power."

The opposition plans to hold street rallies in coming days, and remains determined to stop Nkurunziza from running for a third term, Mugwengezo said.

Nkurunziza's spokesman was not available for comment.