The United States urged east African states to send high-level representatives to Burundi to reiterate their opposition to President Pierre Nkurunziza seeking a third term and to press for free and fair elections.

State Department spokesman John Kirby said the United States opposed Nkurunziza's decision to run for another term, which was in breach of the Arusha agreement that brought an end to the country's civil war in 2005.

"The United States ... encourages high-level visits from Burundi’s neighbors to Burundi to reiterate the importance of upholding the Arusha Agreement and ensuring the conditions necessary for free, fair, and credible elections," Kirby said in a statement.

Nkurunziza's decision has triggered the worst crisis in the small African country since 2005 and raised concerns it could reignite ethnic violence. His opponents say he is violating the constitution, while supporters say a constitutional court ruling allows him to run.

More than 11,000 Burundians - about one percent of the country's population - have already fled across the border for fear violence will spread outside the capital.

Kirby said Washington supported the outcome of a summit of African leaders in Tanzania on Sunday which called for a delay of at least a month and a half in the poll.

The summit of the East African Community included Rwanda, Kenya, Tanzania, Uganda and Burundi, and was joined by South Africa's President Jacob Zuma.