A surgeon from Sierra Leone who was being treated for Ebola in the US has died, a Nebraska hospital announced.

Martin Salia, who has US residency and is married to an American, arrived for treatment in the state on Saturday, in a critical condition.

And on Monday morning the hospital said the 44-year-old had died.

More than 5,000 people have died in the current Ebola outbreak - almost all of them in West Africa. Dr Salia is the second person to die in the US.

Liberian national Thomas Eric Duncan died in Dallas last month after contracting the virus in Monrovia.

The national state of emergency in Liberia ended last week and on Sunday, President Ellen Johnson Sirleaf said she hoped the country would be free of the virus by Christmas.

Dr Salia was in a critical condition when he arrived at the hospital in Omaha, Nebraska, unable to walk.

"We are extremely sorry to announce that the third patient we've cared for with the Ebola virus, Dr Martin Salia, has passed away as a result of the advanced symptoms of the disease," said the Nebraska Medical Center in a statement.

He was extremely ill, said Dr Phil Smith, medical director of the biocontainment unit. "And unfortunately, despite our best efforts, we weren't able to save him," he added.

Dr Salia initially displayed symptoms a week ago but tested negative for the virus, which is "not an unusual event", according to Dr Smith.

A subsequent test indicated he was infected with the virus, but US healthcare professionals were not able to say why it took nearly a week following diagnosis to transport him to the US.

Dr Salia's wife, Isatu Salia, said she was appreciative of the treatment her husband had received at the US facility.

The White House sent condolences and said his death "is another reminder of the human toll of this disease and of the continued imperative to tackle this epidemic on the front lines, where Dr Salia was engaged in his calling".

He had worked as a general surgeon at Kissy United Methodist Hospital in the Sierra Leone capital, Freetown.

It is not known whether he was involved in the care of Ebola patients.

He was suffering from advanced symptoms, including kidney and respiratory failure, when he landed on US soil and was taken to the hospital in Omaha.

Two other Ebola patients were successfully treated at the unit, which is one of only a handful of specialist treatment centres in the US.