The leader of the World Health Organization said Tuesday that he's “deeply concerned about the scale and speed" of the Ebola epidemic in Africa.

The comment by WHO Director-General Tedros Adhanom Ghebreyesus amid a report by the Congolese Health Ministry that found 531 suspected infections of the deadly virus and that at least 131 people have died, according to NBC News.

An American missionary was among those who tested positive for the virus. 

Global health experts worry about the potential for the outbreak to cause widespread illness and death in Africa, in part because the cases involve a rare strain that has no approved vaccine or treatment. There is also a shortage of available tests. 

In addition, the outbreak went undetected for weeks, which is making it difficult to contain. The U.S. State Department is "strongly" warning Americans not to travel to Congo, South Sudan or Uganda for any reason.