The facilitator of talks in Burundi aimed at ending the unrest caused by the president's controversial bid for a third term says government representatives failed to show up, forcing the mediation process to be adjourned just days ahead of Tuesday's presidential polls.

Ugandan Defense Minister Crypus Kiyonga, who is acting as mediator, stopped short of saying the talks had collapsed and said the government should be given a chance to explain itself.

Earlier United Nations-backed negotiations between the parties collapsed after the opposition insisted on President Pierre Nkurunziza's exit.

Burundi has been rocked by violence that has left more than 100 people dead and over 144,000 refugees have fled the country since Nkurunziza announced his bid for a third term in April.