Dr. Martin Salia, who has US residency and is married to an American citizen, has been taken to a hospital in Nebraska.
Meanwhile France has advised its citizens not to travel to parts of Mali, following the deaths of three people there from the disease.
More than 5,000 people have been killed in the current Ebola outbreak – almost all of them in West Africa.
Sierra Leone is one of the three worst affected countries, along with Liberia and Guinea.
Dr. Martin Salia, 44, had been working as a general surgeon at Kissy United Methodist Hospital in the Sierra Leone capital, Freetown.
It is not clear whether he was involved in the care of Ebola patients.
The doctor, a Sierra Leone citizen who lives in the US state of Maryland, tested positive for the virus on November 10.
After being deemed stable enough to fly, he arrived in Omaha on Saturday afternoon, November 15, and was transferred to an isolation unit at the Nebraska Medical Center.
“Information coming from the team caring for him in Sierra Leone indicates he is critically ill – possibly sicker than the first patients successfully treated in the United States,” the Nebraska hospital said in a statement.
Dr. Martin Salia is the 10th person treated for Ebola in the US. All but one – a Liberian man named Thomas Eric Duncan – have recovered.
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