Exxon Mobil Corp has seen some of its oil and gas activities in West Africa disrupted by the Ebola outbreak, including plans to drill offshore Liberia, the company’s chief executive officer said on Thursday.

Exxon, the world’s largest publicly traded oil company which has operations in Nigeria and Liberia, is prohibiting some employees from traveling to the countries directly affected by the disease, and is taking precautionary measures related to workers’ families, executive said.

The virus has killed at least 3,338 people in West Africa in the worst such outbreak on record. Earlier this week, the first person was diagnosed with Ebola in the United States, in Dallas.

“We had some drilling plans for some blocks offshore West Africa in Liberia,” CEO Rex Tillerson told a news conference to announce $18 million in grants to three Houston hospitals. “We are having to look at when it would be prudent to resume operations there because you do have to have shore-based support.”

The virus has hit hardest in Liberia, but also struck Sierra Leone and Guinea, and affected Senegal and Nigeria.

The response, part of Exxon’s plan to combat “pandemics,” has so far been “fairly low level,” Tillerson said.

Last year in April, Exxon said its Liberian affiliate had acquired an 80 percent interest in Liberia Block 13, located offshore the country. Canadian Overseas Petroleum Ltd is a 20 percent partner in the project.

Exxon through its affiliates drills offshore Nigeria, where it holds nearly 1 million acres and also makes petroleum products, according to the Irving, Texas, company’s annual filing with the U.S. Securities and Exchange Commission.

Exxon has restricted non-essential travel to Nigeria, a spokesman said in an email.

Shares of Exxon rose nearly 1 percent to $93.68 in afternoon New York Stock Exchange Trading. (Editing by Terry Wade and Jeffrey Benkoe)