Vote-counting has started in Burundi's referendum on constitutional changes that would extend the president's rule until 2034.

Five million Burundians were registered to vote in Thursday's referendum that raised concerns about further bloodshed in the East African country that has seen deadly political violence since 2015.

It is not clear when final results will be announced.

President Pierre Nkurunziza had campaigned forcefully for the changes that include extending the president's term from five years to seven. That could give him another 14 years in power when his current term expires in 2020.

Opposition leaders say Nkurunziza's stay in power is already unconstitutional. His quest for a disputed third term in 2015 led to violent protests that have killed at least 1,200 people and sent hundreds of thousands fleeing the country.