NAIROBI, Kenya, Sept. 11 — United Nations officials reported an outbreak of Ebola virus in central Congo today and were rushing supplies and doctors into the region to contain the outbreak. But they said they did not know if Ebola was the cause of a mystery illness that has killed more than 100 people in the same area.

According to Gregory Hartl, a spokesman for the World Health Organization, five samples taken from sick people in the area have tested positive for Ebola, but another disease may be involved because other patients have been responding to antibiotic medicine, which would not work on a virus like Ebola.

“It wouldn’t be surprising if something else was going on,” Mr. Hartl said. “But because Ebola is involved, we have to be on high alert.”

Ebola is fast-acting and highly contagious, and is one of the deadliest diseases on the planet, killing more than half the people who become infected. In severe cases, victims hemorrhage all over their body and bleed from their orifices before dying.