US oil giant ExxonMobil said Thursday it has limited employee travel to countries hit by the deadly Ebola virus.

“We have restricted non-essential travel to countries affected by Ebola,” Exxon spokesman Patrick McGinn said in an email without giving specifics.

Exxon is exploring for oil in Liberia, one of the three countries severely hit by the disease. The others are Guunea and Sierra Leone.

It also has significant activities in Nigeria, which has seen about 20 confirmed Ebola cases but so far appears to have the disease under control.

Exxon was not clear on whether the travel ban included Nigeria.

Exxon made the decision two days after the first case of Ebola was diagnosed inside the United States, of a man who traveled to the country from Liberia in September.

The man is being treated in a hospital in Dallas, Texas where the Exxon headquarters is located.

On Thursday, Texas health officials said they were monitoring 100 people who possibly had contact with the man for signs of the disease, and ordered four close family members to stay home.

The latest outbreak of the Ebola virus has already ki