The head doctor fighting the deadly tropical virus Ebola in Sierra Leone has himself caught the disease, the government said.

The 39-year-old Sheik Umar Khan, hailed as a "national hero" by the health ministry, was leading the fight to control an outbreak that has killed 206 people in the West African country. Ebola kills up to 90 percent of those infected and there is no cure or vaccine.

Across Guinea, Liberia and Sierra Leone, more than 600 people have died from the illness, according to the World Health Organisation, placing great strain on the health systems of some of Africa's poorest countries.

Khan, a Sierra Leonean virologist credited with treating more than 100 Ebola victims, has been transferred to a treatment ward run by medical charity Medecins Sans Frontieres, according to the statement released late on Tuesday by the president's office.

Health Minister Miatta Kargbo called Khan a national hero and said she would "do anything and everything in my power to ensure he survives".

Khan told Reuters in late June that he was worried about contracting Ebola. "I am afraid for my life, I must say, because I cherish my life," he said in an interview, showing no signs of ill health at the time.

"Health workers are prone to the disease because we are the first port of call for somebody who is sickened by disease. Even with the full protective clothing you put on, you are at risk."

Three days ago, three nurses working in the same Ebola treatment centre alongside Khan died from the disease.

The Ebola outbreak started in Guinea's remote southeast in February and has since spread across the region. Symptoms of the highly infectious disease are diarrhea, vomiting and internal and external bleeding.