The first person diagnosed with Ebola in the United States was fighting for his life at a Dallas hospital on Sunday and appeared not to be receiving any of the experimental medicines for the virus, a top U.S. medical official said.

Thomas Eric Duncan became ill after arriving in the Texas city from Liberia two weeks ago, heightening concerns that the worst Ebola epidemic on record could spread from West Africa, where it began in March. The hemorrhagic fever has killed at least 3,400 people out of at least 7,490 probable, suspected and confirmed cases.

"The man in Dallas, who is fighting for his life, is the only patient to develop Ebola in the United States," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said on CNN's "State of the Union."