A third top Ebola doctor has died in Sierra Leone, a government official in the west African nation has confirmed. The news came as another scientist, who also contracted the disease there, began treatment in Germany on Wednesday.

Health workers are trying to work out how the scientist, whose identity and condition are being withheld for privacy reasons, contracted Ebola before his overnight evacuation to a Hamburg hospital, it has been reported.

"The international surge of health workers is extremely important and if something happens, if health workers get infected and it scares off other international health workers from coming, we will be in dire straits," said World Health Organisation spokesman Christy Feig.

The WHO said the stricken man was from Senegal and was infected while working for the agency as a consultant.

Dr Stefan Schmiedel, who is helping oversee his treatment at the University Medical Centre Hamburg-Eppendorf, said the clinic would not be using experimental medicine, instead concentrating on "supportive care" such as fever reduction and fluid management.

"In west Africa the patients die relatively quickly of the illness, or survive and then return to health," he said. "How that will go under our medical supervision, we can't yet estimate."

The latest medic to die from the virus was named as Dr Sahr Rogers. He was working at a hospital in the eastern town of Kenema when he contracted Ebola, according to Sierra Leonean presidential adviser Ibrahim Ben Kargbo.

His death raises fresh concerns over the country's ability to fight Ebola, which has killed more than 1,400 people across west Africa.

On Tuesday, the WHO said it was pulling out its team from the eastern Sierra Leonean city of Kailahun. According to the AP, the agency's representative said the team was exhausted and the added stress caused by one of their colleagues contracting Ebola risked increasing the chances of mistakes being made.

Canada also announced it was evacuating a three-member mobile laboratory team from Sierra Leone after people in their hotel were diagnosed with Ebola.

Care workers are at increased risk of contracting the disease due to their proximity to patients. To date, the WHO has reported more than 240 of them developing Ebola in Guinea, Liberia, Sierra Leone and Nigeria. More than half of that number have died.

In Liberia, human rights campaigners have demand an investigation into the death of a teenager they said was shot by soldiers during a riot at a Monrovia slum that had been quarantined in a bid to stop the spread of the virus. According to Agence France-Press, 16-year-old Siafa Kamara was one of four residents of the West Point slum wounded during violent clashes last week. His family announced on Sunday that he had died in hospital after sustaining bullet wounds, although the government has denied he was shot, saying he injured his legs on barbed wire.

"There is a need to set up an independent body to investigate the death of the boy. You cannot fight Ebola with the army," Gladys Johnson, head of the independent national commission on human rights said on Liberian radio.

In Wales, a doctor quarantined for three weeks after helping Ebola patients in Africa has returned to work, it was reported.

Specialist registrar Nathalie MacDermott, who works at the neonatal unit in Singleton hospital in Swansea, flew to Liberia with Christian organisation Samaritan's Purse to help victims of the deadly virus.