* British man to be treated at isolation unit in London
* World Health Organization confirms WHO worker infected
* Sierra Leone source says WHO worker is Senegalese (Adds details)
By Josephus Olu-Mammah and Kylie MacLellan
FREETOWN/LONDON, Aug 24 (Reuters) - A British healthcare worker who contracted Ebola in Sierra Leone - the first Briton to catch the deadly virus - was flown home for treatment on Sunday, as the World Health Organization confirmed another foreign medic had caught the disease.
Britain's Foreign Secretary Philip Hammond authorised the repatriation of the male medical worker - whose identity has not been disclosed - after he was analysed by doctors from Britain and Sierra Leone.
The worst ever outbreak of the hemorrhagic fever has so far killed at least 1,427 people, mostly in Sierra Leone, Liberia and neighbouring Guinea. Five deaths have been reported in Nigeria.
Britain's Deputy Chief Medical Officer John Watson said final approval for the evacuation was given on the ground in Sierra Leone by a team of physicians who had arrived on a specially equipped Royal Air Force cargo plane.
The Boeing C-17 left the Sierra Leonean capital Freetown bound for Britain at around 1250 GMT.
"We understand that this patient, during the course of the work that he was carrying out, was exposed about a week ago and became unwell two or three days ago," Watson told Sky News.
"The patient is not currently seriously unwell," the UK Department of Health said in a statement.
Upon arrival at the RAF Northolt air base in Britain, he will be transported to an isolation unit at the Royal Free Hospital in London, the department said.
The hospital has the only high-level isolation unit for treatment of infectious diseases in Britain and has a team of specially trained staff.
"Protective measures will be strictly maintained to minimise the risk of transmission to staff transporting the patient to the UK and healthcare workers treating the individual," Paul Cosford, director for health protection at state body Public Health England, said in a statement.
Two U.S. doctors, who contracted Ebola in Liberia and were evacuated to the United States, left hospital last week after receiving treatment with an experimental drug, ZMapp. It was not clear what role the pharmaceutical played in their recovery.
Three African healthcare workers have also improved since receiving ZMapp in Liberia. Its U.S.-based manufacturer, Mapp Biopharmaceutical, has said limited supplies of the drug have already been exhausted.
FIRST WHO WORKED TAKEN ILL
The World Health Organization said one of its healthcare workers had tested positive for Ebola for the first time in Sierra Leone. The WHO said it was working to ensure that the foreign worker, who it did not identify, was receiving the best possible care, including the option of medical evacuation.
A government source in Sierra Leone, who asked not to be identified, said the worker was a Senegalese expert working for the WHO in the eastern town of Kailahun.
The WHO has deployed nearly 400 people from its own staff and partner organisations since the outbreak was detected in March deep in the forest region of southeast Guinea.
In the past six months of the outbreak, more than 225 health workers have fallen ill and nearly 130 have lost their lives to the disease, the WHO said.
It is the first outbreak of the disease in West Africa and the worst since it was discovered in 1976 in the jungles of Democratic Republic of Congo, then known as Zaire.
The WHO is due to release next week details of a draft strategy to combat the disease in West Africa. The U.N. agency has faced criticism that it moved too slowly to contain the outbreak.
With the healthcare systems of Sierra Leone and Liberia already fragile following a decade of civil war in the 1990s, and still lacking staff, the WHO said a surge in foreign healthcare workers was essential.
Senior United Nations System Coordinator for Ebola David Nabarro said on Friday the strategy would involve increasing the number of foreign and national health workers fighting the disease.
Nabarro visited Sierra Leone on Sunday, where he was due to see new laboratory and treatment centre in Freetown. (Additional reporting by Umaru Fofana in Freetown and Daniel Flynn in Dakar; Writing by Daniel Flynn; Editing by Robin Pomeroy)