The Peace Corps on Wednesday removed more than 300 volunteers from the countries as the outbreak spread.
Harf also said the U.S. was working on providing assistance to help bring infected Americans back home. Two stricken Americans were listed as being in "stable but grave" condition in Liberia as of Thursday. Leone's top doctor who treated Ebola patients died earlier from the disease.
The CDC is sending 50 disease control experts in the next 30 days to work on infection control, reporting and education about the disease.
The World Health Organization reports that between July 24 and last Sunday, 122 new cases and 57 deaths were reported. Health organizations say there's no vaccine or cure for the Ebola virus.