A state of emergency has now been declared in Sierra Leone as the death toll from the African Ebola outbreak tops 800.

The official figure reached 826 over the weekend, with the latest updates suggesting the outbreak is spinning out of control.

New cases are emerging at an increasing rate and many health authorities are already running out of ways to control the spread.

“The surge in the number of new cases... calls for concentrated efforts by all to address the identified problems such as health facility transmission and effective contact tracing,” the WHO said.

“This outbreak is moving faster than our efforts to control it,” WHO director-general, Margaret Chan said.

“If the situation continues to deteriorate, the consequences can be catastrophic in terms of lost lives, but also severe socio-economic disruption and a high risk of spread to other countries,” she said.

The nations of Guinea, Liberia and Sierra Leone have unveiled new measures to combat the outbreak, calling in joint army forces to enforce quarantines.

Liberia has closed its borders and shut down schools, and Sierra Leone is planning to adopt similar measures in coming days.

Meanwhile, a specially-adapted private jet is ferrying two US aid workers back to their home country for treatment.

The American doctors have flown in a quarantine plane to a facility at Atlanta’s Emory University hospital.

One of the aid workers, Dr Kent Brantly, was seen walking himself out of an ambulance unassisted when he arrived at the advanced isolation ward, which authorities say could be a good sign that he is fit and healthy enough to fight the deadly disease.

Nancy Writebol, a medic who had been treating Ebola patients in Liberia, will be on her way back to the States in coming days.