Burundi's President Pierre Nkurunziza has signed a decree delaying parliamentary and local council elections by one week.

Burundi's President Pierre Nkurunziza delayed by a week local and parliamentary elections set for May 26, after nearly a month of political unrest and a failed military coup in the east African state, his adviser said on Wednesday.

Nkurunziza made the decision after a recommendation by the election commission and following requests from opposition politicians and the international community for a postponement, Willy Nyamitwe told on Wednesday.

Regional heavyweight South Africa called this week for the June 26 presidential election to be postponed indefinitely to allow stability to return.

However, diplomats fear that the longer the crisis drags on, the greater the chance that what is essentially a power struggle could re-open old wounds in a country with a long history of mass ethnic killing between its Hutu majority and Tutsi minority.

A failed coup last week appeared to expose rifts in the military, a key pillar of post-war unity and reconciliation. Nyamitwe denied any splits in the security forces.

"The army is not divided," he told.

Nkurunziza argues that his presidential bid is legitimate since he was appointed to his first term in office by parliament, rather than by a direct vote.