OMAHA, Nebraska (REUTERS) - A surgeon from Sierra Leone, critically ill with Ebola, was flown to a Nebraska hospital for treatment on Saturday, and is sicker than previous patients treated in the United States, medical officials said.

Dr Martin Salia, 44, a permanent US resident, caught Ebola while working as a surgeon in a Freetown hospital, according to his family. He was stable enough to take a flight from West Africa to Omaha, but was too sick to walk off the plane, medical officials said.

He was transferred to a waiting ambulance in an isolation unit called an ISOPOD, a device used in the transportation of a potentially infectious patient, and rushed to Nebraska Medical Centre to begin treatment, a hospital official said.

"Although the patient's exact condition won't be available until doctors here evaluate him after he arrives, 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 hospital said in a statement.

The patient is the third treated for Ebola in the hospital's Biocontainment Unit since the outbreak gained momentum this year in Sierra Leone, Guinea and Liberia.

Dr Salia was chief medical officer at the United Methodist Church's Kissy Hospital when he was confirmed on Tuesday to have contracted Ebola.

His evacuation was at the request of his wife, a US citizen who lives in Maryland and who has agreed to reimburse the US government for any expense, the US State Department said in a statement.

A medical crew with Phoenix Air examined the patient in Sierra Leone before leaving with him en route to the US on Friday night, the Nebraska hospital said.

According to the latest figures from the World Health Organisation, at least 5,177 people have died in the world's worst recorded Ebola outbreak.

Most of the victims have been in Sierra Leone, Liberia and Guinea, where already weak healthcare systems have been overrun by victims of the disease. A total of 570 local health workers have been infected, with 324 dying.

Dr Salia would be the 10th known case of Ebola in the United States. All but one case was treated successfully.

The Nebraska clinic is one of four American hospitals approved by the federal government to treat Ebola.