The vice president of Burundi's election commission has fled the country, sources told AFP on Saturday, in a new blow to government plans to push ahead with next week's first stage of controversial elections.

Spes Caritas Ndironkeye left the central African nation on a flight for neighbouring 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's crisis surrounds President Pierre Nkurunziza's controversial bid for a third term in office, with opposition and rights groups saying the move violates the constitution as well as the terms of a peace deal that ended a 13-year civil war in 2006.

The capital Bujumbura has been hit by weeks of civil unrest which has left at least 30 dead in a major security crackdown, and the crisis intensified earlier this month when a top general staged a failed coup attempt -- increasing fears that the impoverished, landlocked country could be plunged back into widespread violence.

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.