BUJUMBURA (Reuters) - Opposition parties said on Friday they would boycott Burundi's elections in protest at the president's bid for a third term and over concerns voting would not be fair, escalating the African nation's worst political crisis since a civil war ended in 2005.
Highlighting international worries about a region that has a history of ethnic conflict, U.N. Secretary-General Ban Ki-moon said voting should be postponed because he was "deeply concerned over the prevailing political and security environment".
The United States said that it was putting electoral assistance to Burundi on hold because President Pierre Nkurunziza was pressing ahead with an election timetable in the absence of conditions necessary for a credible vote.
A parliamentary election is due to take place on Monday with a presidential vote to follow on July 15. Both have already been pushed back after weeks of protests against Nkurunziza's announcement in April he would stand again.
The U.N. Security Council called on all the Burundian parties to take part in urgent talks focusing on creating conditions conducive to the organization of free, fair, transparent and credible elections.
Burundi's U.N. Ambassador Albert Shingiro told the 15-member council on Friday that the elections would go ahead on Monday.
"Over 95 percent of the population wants to move forward to the election and not remain hostage to this radical minority which does not wish to see elections," he said.
Protesters, dozens of whom returned to Bujumbura's streets on Friday, say another five-year term is unconstitutional. The president cites a court ruling saying he is allowed to run.
Dozens of people have been killed in the unrest and the U.N. refugee agency UNHCR said on Friday almost 127,000 people, more than 1 percent of the population, had fled to neighboring states, many citing fears about election-related violence.
Announcing an election boycott by the group of 17 opposition parties, Francois Nyamoya, secretary general of the MSD party, told Reuters voting would "not be credible".
Frederic Bamvuginyumvira said his Frodebu party and other opposition groups opposed the new vote timetable because it was drawn up without consultation, which the government denies.
He repeated calls for the ruling party's Imbonerakure youth wing to be disarmed, echoing comments from Western and African states. The ruling CNDD-FDD denies the youths have been armed.
A group of eight Burundi civil society groups called in a statement for "all Burundians to systematically boycott elections prepared by Nkurunziza who wants to impose himself as a leader in violation of republic laws."
The government has previously dismissed boycott threats, saying the opponents were scared of being defeated at the polls.
The European Union, a major donor, and African neighbors have all voiced worries that the country was not ready to vote.
European foreign ministers said this week they did not expect conditions for a fair vote to be in place in time, such as re-opening private radio stations closed during the unrest.
Announcing a freeze on U.S. technical assistance and voter education activities, State Department spokesman John Kirby said in a statement, "We are prepared to take additional measures against those individuals responsible for - or complicit in - undermining democracy and the rule of law, and promoting instability in Burundi."
At talks to defuse the crisis, international mediators proposed a delay in local, parliamentary and presidential polls to July 30, but opposition parties stuck to a demand for a three-month postponement, one opposition politician said.
There was no immediate comment from the government or ruling party, which stayed away from the U.N.- and African-mediated talks. The CNDD-FDD said this week it needed to focus on campaigning. The government has promised a fair vote.
Protests on the streets of the capital, where most of the rallies were held, have subsided for the past two weeks.
But dozens of protesters blocked roads with makeshift barriers in a restive Bujumbura district on Friday, watched by police, a Reuters witness said.
For six weeks or so after Nkurunziza had said he would stand, there were almost daily clashes between police and demonstrators, mostly in the capital but some outside.
Tensions have also emerged in government. A vice president, Gervais Rufyikiri, said this week he had fled Burundi after he was threatened for denouncing Nkurunziza's re-election bid, an allegation the government denied.
UNHCR offices in the region "have been noting a steady increase in arrivals of Burundi refugees" as Burundi gears up for voting, the agency's spokesman Adrian Edwards told a news briefing in Geneva on Friday.
Additional reporting by Edmund Blair in Nairobi, Patrick Nduwimana in Kigali, Michelle Nichols at the United Nations, Stephanie Nebehay in Geneva and Mohammad Zargham in Washington; Writing by Edmund Blair; Editing by Andrew Heavens, Hugh Lawson, Chizu Nomiyama, Toni Reinhold