Burundi's second vice president has fled the country and sought refuge in Belgium after urging the president to step down, days ahead of key elections.

"Put the interests of the Burundian people before your personal interests and withdraw your presidential bid, because it violates the constitution," Gervais Rufyikiri said in a letter addressed to President Pierre Nkurunziza. He has told France 24 he had sought refuge in Belgium after opposing Nkurunziza's bid for a third term in power.