The vice president of Burundi's election commission, Spes Caritas Ndironkeye, has fled the country before the coming elections as protests and violence continue to hit Burundi.
Ndironkeye left the central African nation on a flight for neighboring Rwanda on Friday evening, associates said, adding that she had left behind a letter of resignation.
An electoral commission source said she "left without saying goodbye, without saying where she was going." A second member of the five-person commission, Illuminata Ndabahagamye, is also thought to have fled, sources said.
"What has happened is a catastrophe, but it was inevitable," another commission source said.
"Technically, the Election Commission can continue to work with four out of five members. But if two have left, no decision can be taken and it will be impossible to replace them before June 5," the source said.
Burundi has been hit by weeks of civil unrest as the opposition wants Nkurunziza to withdraw his third-term bid in the upcoming June 26 elections, claiming it violates the country's constitution and the Arusha Accords, a peace deal that ended an ethnic civil war in 2005.
The constitution limits the presidency to two terms in office, but Nkurunziza's supporters say he can run again because his first term was picked by lawmakers and not by elections.
The opposition has also said the holding of free and fair elections is impossible, with independent media silenced and allegations of threats and intimidation by Nkurunziza's supporters.
Parliamentary elections are due to be held on June 5, with a presidential poll scheduled for June 26.
The US placed visa restrictions on Burundians it deemed responsible before the elections, claiming that violence and militias have no place in the democratic process and all people should vote for a better Burundi.