Over 50,000 Burundians have fled their country since the start of political violence in April, the UN refugee agency (UNHCR) said on Friday, adding that many more were trying to leave but faced hurdles.

UNHCR spokesperson Adrian Edwards said more than 25,000 had crossed into Rwanda, another 7,700 into Tanzania in a week, and 8,000 people had gone to the South Kivu province of the Democratic Republic of Congo.

"In all these cases women and children, including a large number of unaccompanied children, are in the majority," he said.

A third term bid by Burundi's president has sparked violent demonstrations claiming at least 18 lives and sparking global concern that the central African nation - which endured a brutal 13-year civil war - could lapse into conflict again.

Opposition parties and civil society groups say President Pierre Nkurunziza's third-term bid in elections due in June violates both the constitution, which limits a president to two terms in office, and the accords that in 2006 ended the civil war between Tutsis and Hutus.