WASHINGTON (AFP) - An American doctor battling West Africa's Ebola epidemic has himself fallen sick with the disease in Liberia, his aid agency said.

Samaritan's Purse, a Christian charity, said Dr Kent Brantly had been isolated at the group's Ebola treatment center at the ELWA hospital in the Liberian capital Monrovia. He was in stable condition on Sunday, the aid agency said. His symptoms include intermittent fever and body aches.

"We remain optimistic for his recovery, but certainly he is not out of the woods," said spokesman Melissa Strickland.

"Dr Brantly is married with two children," the group said, in a statement posted to its website on Saturday.

"Samaritan's Purse is committed to doing everything possible to help Dr Brantly during this time of crisis. We ask everyone to please pray for him and his family."

Dr Brantly is the medical director of the Samaritan's Purse Ebola case management center in Liberia, where the agency continues to work with Liberian and international health officials to contain the outbreak. He began work with the group's Liberia team in October and had focused on Ebola patients since June.

The US State Department said it was aware of an Ebola case but could not provide information about a private individual.

The news comes after a doctor in charge of an Ebola treatment centre in Sierra Leone was admitted to hospital after contracting the deadly virus himself, the country's Health Ministry said last Wednesday. News of his infection in turn came just days after three nurses from the same facility died of the disease.

Dr Omar Khan was in charge of the main Ebola treatment centre in Kenema, around 320km east of the capital Freetown, when he caught the virus.

Health Minister Miatta Kargbo called Dr Khan a "national hero", praising his "tremendous sacrifice".

"I will do anything and everything in my power to ensure he survives. I call on all Sierra Leoneans to remember him in their thoughts and prayers," he said.

Dr Khan has been transferred to another treatment facility run the medical charity Doctors Without Borders (MSF), the Health Ministry said.

"He is responding to treatment and is very cheerful and fine," Dr Abubakarr Fofanah, the deputy health minister, told AFP by phone.

The news of Dr Khan's illness came as the ministry confirmed the deaths of three nurses last Monday, describing them as "veterans who have spent over 15 years working in infectious diseases and have saved the lives of more than 100 people".

"The last few days have been challenging to us at the ministry. These people were a source of hope and inspiration," spokesman Yahya Tunis told journalists on Wednesday.

Nurses at the Kenema Ebola centre, located within a government-run hospital, are now calling for it to be relocated to another site and for MSF to take over the running of the facility, Mr Tunis added.

Ebola is an haemorrhagic fever with a very high fatality rate. Liberia, Sierra Leone and Guinea have borne the brunt of the recent epidemic, and last week Nigeria recorded its first death.

As of July 20, the number of Ebola cases recorded in the months-long epidemic stood at 1,093, including more than 660 deaths, according to the World Health Organisation.

It said 28 new deaths were recorded between July 18 and July 20. Thirteen were in Sierra Leone, 11 in Liberia and four in Guinea, which had previously borne the brunt.

Forty-five new cases were recorded over the same period.

Africa's most populous country Nigeria on Friday became the fourth west African country to be affected by the virus, announcing that a Liberian national in guarantine in a Lagos hospital had died of the disease.

The Health Ministry said last Friday that a 40-year-old Liberian man died at a private hospital in Lagos from the disease.

The victim, who worked for the Liberian government, collapsed at Lagos international airport after arriving on a flight from Monrovia via the Togolese capital Lome on Tuesday, according to the Nigerian government.

He did not come into contact with the public in Lagos other than health workers before being isolated at the hospital, they added.

The World Health Organisation (WHO) said on Saturday that efforts were being made to track down people the victim may have come into contact with on his journey from the Liberian capital.

"Contact tracing under way - Liberia, Nigeria, Togo. WHO deployed staff to Nigeria to help w/ this & safe handling of remains," the global health body's African region said on its Twitter account @WHOAFRO.Nigerian Health Minister Onyebuchi Chukwu had previously said all passengers who travelled with the victim on the Togo-based carrier ASKY from Lome had been traced.

Meanwhile an emergency operations centre has been set up, coordinated by the Nigeria Centre for Disease Control (CDC), and all hospitals have been equipped to handle emergencies and suspected cases, the minister added.

Borders have not been closed but health specialists have been deployed to all sea ports and international airports to identify any passengers displaying symptoms associated with Ebola.

In Lagos, Mr Yewande Adeshina, the special adviser on public health to the Lagos State governor, said: "We have taken delivery of 30 personal protective equipment (sets) from the federal ministry of health and they have promised to give an additional 220 for our medical personnel to protect them from any risks.

"We are also reaching out to private clinics, hospitals and health facilities in the state to promptly report any suspected case to the authorities."

Information Minister Labaran Maku said a radio and television public awareness campaign about Ebola has been running for several months on state media and would continue.

Although Guinea recorded the lowest number of new cases - five - it still has the highest death toll.

In total, Guinea has seen 314 fatalities and 415 cases since the outbreak began in January.

Sierra Leone's case-count has now overtaken Guinea's, however. It reported 12 new cases, taking its total to 454, with 219 deaths.

Liberia reported 28 new cases, lifting its total to 224. Of those, 127 have been fatal.

The virus can fell victims within days, causing severe fever and muscle pain, vomiting, diarrhoea and, in some cases, organ failure and unstoppable bleeding.

Ebola is believed to be carried by animals hunted for meat, notably bats.

It spreads among humans via bodily fluids including sweat, meaning you can get sick from touching an infected person.

With no vaccine, patients believed to have caught the virus must be isolated to prevent further contagion.

Ebola first emerged in 1976 in what is now the Democratic Republic of Congo, and is named after a river there.

It is one of several viruses responsible for haemorrhagic fever.

No medicine or vaccine exists for the tropical virus, named after a small river in the DR Congo.

Five "species" of Ebola have been identified so far, and have been named Bundibugyo, Sudan, Zaire, Tai Forest and Reston.

The first three are particularly dangerous, with fatality rates of up to 90 per cent.

The Reston species has also been identified in China and the Philippines, but no associated deaths have been reported in those countries to date.

Ebola causes severe fever and muscle pain, weakness, vomiting and diarrhoea - in some cases shutting down organs and causing unstoppable bleeding.

It is a so-called filovirus, transmitted through contact with the blood, body fluids, secretions or organs of an infected person.

Experts say that while extremely virulent, the virus can be contained because it kills victims faster than it spreads.

The incubation period between exposure and the first symptoms varies from two to 21 days.

The virus has been known to spread at burials where mourners touch the body, but doctors and nurses have also fallen ill after failing to take adequate precautions.

Even testing blood specimens for the disease presents "an extreme risk", the WHO has warned, and is done only in the strictest containment conditions.

The virus's natural host in Africa is thought to be a species of rainforest bat, while another concentration has been found in the western Pacific region.

People have contracted the virus after handling both dead and living chimpanzees, gorillas, monkeys, forest antelope and porcupines.

For now, the only approach is to isolate patients and promptly bury the dead, says the WHO.

Hospital staff should use gloves, masks and goggles, and disinfect religiously.

"Several potential vaccines are being tested but it could be several years before any are available," according to a WHO factsheet.

"A new drug therapy has shown some promise in laboratory studies and is currently being evaluated."