The U.S. envoy to Africa's Great Lakes region is urging Burundi's government and the opposition to start internationally mediated talks without pre-conditions in order to end the country's political crisis.

Tom Perriello told reporters in the Burundian capital, Bujumbura, that the international community is keenly watching events in Burundi, where a series of extrajudicial killings have raised concern.

A regional bloc nominated Ugandan President Yoweri Museveni to mediate the crisis but the talks have not started.

Perriello said he is visiting Burundi to express Washington's "grave concerns" over the deteriorating political situation in Burundi, which has been hit by violence since April following the president's controversial decision to seek a third term in office.

At least 240 people have been killed since April, according to the U.N.