An American photojournalist who contracted Ebola while working in West Africa has begun his journey home for treatment, while a man who recently arrived in Dallas from Liberia remains in critical condition with the disease.

Ashoka Mukpo, 33, will be the second Ebola patient to be treated at the Nebraska Medical Centre's specialised isolation unit.

Mukpo was working as a freelance cameraman for NBC News in Liberia when he became ill last week.

NBC reported on Sunday evening that Mukpo had started his journey to the US for treatment and that he would arrive on Monday morning.

Mukpo's family said on Friday he would be treated in Omaha. Hospital officials said they expected an Ebola patient to arrive on Monday, but declined to provide a name.

He is the fifth American to return to the US for treatment since the start of the latest Ebola outbreak, which the World Health Organisation estimates has killed more than 3,400 people.

The hospital's biocontainment unit was created in 2005 specifically to handle illnesses like this, Dr Phil Smith, who oversees the unit, said.

"We are ready, willing and able to care for this patient," Smith said. "We consider it our duty to give these American citizens the best possible care we can."

'Counting the minutes'

Mukpo's father, Dr Mitchell Levy, told NBC on Sunday that his son was "counting the minutes" until he could leave Liberia but that he was not feeling that ill on Sunday.

Levy said the family was travelling from Rhode Island to Nebraska.

Doctors at the isolation unit - the largest of four nationwide - would evaluate Mukpo when he arrives before determining how to treat him.

The virus that causes Ebola is not airborne and can only be spread through direct contact with bodily fluids - blood, sweat, vomit, faeces, urine, saliva or semen - of an infected person who is showing symptoms.

Doctors said they would apply the lessons learned while treating American aid worker Rick Sacra in September.

Sacra was successfully treated in the Nebraska unit and was allowed to return to his home in Massachusetts after three weeks, on September 25.

"Truly, focusing on symptom management is key with these patients," Dr Rosanna Morris said on Friday.

Sacra received an experimental Tekmira Pharmaceuticals drug called TKM-Ebola, as well as two blood transfusions from another American aid worker who recovered from Ebola at an Atlanta hospital.

The transfusions are believed to help a patient fight off the virus because the survivor's blood carries antibodies for the disease.

Sacra also received supportive care, including IV fluids and aggressive electrolyte management.

Symptoms of infection

Meanwhile in Dallas, Thomas Eric Duncan, who has been hospitalised at the Texas Health Presbyterian Hospital for one week, was listed in critical condition on Sunday.

Dr Tom Frieden, director of the federal Centres for Disease Control and Prevention, said he was aware that Duncan's health had "taken a turn for the worse", but he declined to describe Duncan's condition further.

Duncan arrived in Dallas from Liberia on September 20 and fell ill a few days later.

Four members of a family who hosted Duncan in their northeast Dallas apartment are being kept in isolation, though they have not shown symptoms of infection.

Ten people definitely had close contact with Duncan and a further 38 may have been around him when he was showing symptoms of the disease, officials said.

Earlier on Sunday, authorities found a homeless man who may have had contact with Duncan, Sana Syed, the Dallas city spokesperson, said.

Clay Jenkins, a Dallas county judge, called the man, whom he did not identify, a "low-risk individual", and said he would be placed in housing that would allow health workers direct access to him daily.