ATLANTA—Two Americans infected with Ebola in Liberia—one in a hospital here and another set to arrive Tuesday—were treated in that West African country with an experimental drug not yet evaluated for safety in humans.

It is too early to know if the drug, developed by a U.S. company, has been successful, said Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases.

Dr. Fauci added Monday that a clinical trial that includes more patients would be needed to demonstrate the drug's efficacy and safety.

There is no approved vaccine or treatment for Ebola, a deadly disease that has been spreading through the West African nations of Liberia, Guinea and Sierra Leone. It is the largest outbreak of the disease in history.

In New York City on Monday, Mount Sinai Hospital officials said they were testing a man for Ebola who recently returned from West Africa with a high fever and gastrointestinal distress. Authorities, who say the man is in isolation, wouldn't identify him or the country he had visited.