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 has raised concern.
A regional bloc nominated Ugandan President Yoweri Museveni to mediate the crisis but the talks have not started. Perriello 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.
Last week President Pierre Nkurunziza urged the security forces to use all means necessary to restore order. But many residents charge it is the security forces that are carrying out the killings. Burundi's security forces are currently disarming parts of Bujumbura, an exercise that has been marred by violence. The police have been accused of unfairly targeting some people, and many residents have fled their homes fearing they will be framed and jailed.
Nkurunziza took power in 2005 near the end of a civil war in which some 300,000 people were killed between 1993 and 2006.