A doctor infected with Ebola in Sierra Leone arrived in the United States on Saturday to be treated for the deadly virus.

The University of Nebraska Medical Center tweeted that Martin Salia, a legal US resident, had "safely arrived" in Omaha after a flight from Sierra Leone's capital Freetown.

He was then taken by ambulance to the center's biocontainment unit, where he will be kept in isolation to avoid infecting others.

Salia is the third Ebola patient to be treated by the UNMC, one of a handful of medical facilities in the United States specially designated to treat Ebola patients. Both of the previous patients survived.

He had traveled back to his home country to treat Ebola patients at Freetown's Connaught Hospital amid a devastating outbreak when he became infected.

His evacuation was facilitated by the US government at the request of his wife, an American citizen who agreed to reimburse all expenses, State Department spokeswoman Jen Psaki said in a statement.

"Just as we have done in previous cases, every precaution is being taken to ensure the evacuation is completed safely and securely, that critical care is provided en route, and that strict isolation is maintained," Psaki said.

Doctors have said they were told that Salia is "critically ill."

They said his exact condition would be evaluated when he arrives, but that he is "possibly sicker than the first patients successfully treated in the United States."

"We immediately started preparing the unit and notifying staff members of this possibility," biocontainment unit medical director Phil Smith said in a statement.

"We've obviously been through this a couple of times before so we know what to expect."

CNN has reported that Salia has several children.

There are currently no cases of Ebola in the United States, where nine people have been treated for the killer virus. Only one -- Liberian-born Thomas Eric Duncan -- has died from the disease on US soil.

The cases are part of the deadliest Ebola outbreak ever, which has killed more than 5,100 people in West Africa and infected nearly 15,000 in total, mostly in hardest hit Liberia, Sierra Leone and Guinea.

Despite some hopeful signs -- Liberia has lifted its state of emergency and the DR Congo announced the end of its own, unrelated, outbreak of Ebola -- the recent deaths of three people in Mali have fueled fears of a new African hotspot.

There is no known cure for Ebola, one of the deadliest known pathogens, but trials for several possible treatments were announced this week in West Africa and Canada. The disease is spread through contact with bodily fluids.