"This decision is non-negotiable," government spokesman Philippe Nzobonariba said in a radio broadcast, dismissing opposition demands that the president should step down.
The government said the electoral commission's proposal to delay the presidential poll until July 15 was the final time polls would be postponed.
Non fewer than 40 people have died and scores injured in protests that began when Mr Nkurunziza announced in late April that he would run a third time for presidency after Burundi's constitutional court gave him the green light.
