A British military healthcare worker who tested positive for Ebola in Sierra Leone is being flown back to Britain and four others are being assessed, officials in London said on Thursday.

The unnamed healthcare worker who has contracted the disease was being transported in a specially equipped medical plane and will be treated in an isolation unit at the Royal Free Hospital in London, the country's designated Ebola treatment center.

On board the plane are two other military healthcare workers who have not been diagnosed with Ebola but were found to require further assessment because of contact with the diagnosed person. They will be monitored for symptoms at the Royal Free.

Two other military healthcare workers are being assessed in Sierra Leone before a decision is made on whether to evacuate them back to Britain.

The healthcare worker who has tested positive was exposed to the virus in a frontline care facility for Ebola patients.

Britain, the former colonial ruler of Sierra Leone, currently has between 600 and 700 military personnel deployed in the West African country to help combat the virus.

Ebola has now killed nearly 10,000 people in the three worst-affected countries, Sierra Leone, Liberia and Guinea.

Rates of new infections have come down swiftly in recent months, however. Liberia last week released its last known Ebola patient from hospital, but Sierra Leone still had 127 patients in Ebola treatment centers as of March 10.